A poster of the Hindi film “Vicky Donor” at Dr. Himanshu Roy’s fertility clinic in Patna, Bihar.

India’s caste system, in which people are born into a certain group, is now asserting itself even before birth as more couples in rural areas turn to sperm donors as treatment for infertility.

In Patna, the capital of Bihar, one of the most populous states, Dr. Himanshu Roy runs a popular fertility clinic that also has a sperm bank for patients. In the huge hall that leads to his office, a poster of “Vicky Donor,” a recent Bollywood comedy about a sperm donor, greets the crowds of hopeful couples.

Dr. Roy said the demand in Bihar for sperm donations in recent years has outstripped supply as more rural couples shed their social inhibitions to visit clinics like his. “And, yes, the film ‘Vicky Donor’ too has played its part,” he said.

Naturally, couples using donated sperm want to know all they can about the donor. In many parts of the world, that would include where a donor went to school, his medical history or what kind of job he had. In Bihar, couples’ primary concern is caste, doctors said.

“Sometimes, before asking about the culture, health, physical features, even religion, they ask about caste of the donor, and it becomes very difficult not to satisfy them,” said Dr. Saurabh Kumar, who runs Frozen Cell, another sperm bank in Patna. He reveals the donor’s caste if patients ask.

A gynecologist who runs a fertility center in the adjoining district of Muzaffarpur said that patients can get “fanatical” about knowing the caste of the donor.

“It’s almost like tubes in our sperm banks are labeled with ‘Brahmin,’ Bhumihar,’ ‘Yadav,’ ‘Rajput’ and several other caste stickers,” said the doctor, who asked to remain anonymous because of the sensitivity of the issue. In Bihar, caste plays a large role in society and culture, but many people are reluctant to discuss caste preferences openly out of concern that it would make them appear to be prejudiced.

In a rural state like Bihar, the emphasis on “purity” is driving such inquiries about a sperm donor’s caste, said S.N. Sinha, a sociologist.

“They might think that right caste would ensure the bloodline stays pure,” Mr. Sinha said.

About 40 of Frozen Cell’s donors come from Patna, said Dr. Kumar. “Most of them are college-going students living in hostels or lodges,” he said.

His clinic pays 300 to 500 rupees ($5.40 to $9) each time men donate. “The payment depends on the quality of their sperm,” Dr. Kumar said.“Usually, they donate for buying cellphone recharge coupons or to buy petrol in their bikes,” he said. “But they’re very particular about their anonymity.”

One Frozen Cell sperm donor, a university student who asked to remain anonymous because he doesn’t want anyone to know he donates, said he receives 500 rupees each time he visits the sperm bank. “It supports me financially,” he said. “I donate every week. It is as normal as a blood donation.”

He has persuaded two of his young friends to donate as well, he said.

“Initially, when I told them, they laughed at me, but when I convinced them about its benefits they readily agreed, and since then they too have become regulars,” he said.

Another sperm donor, a 25-year-old bank employee, who also asked not to be identified, said, “First I did it for fun, but now I do it for money.”

“Invest koi nahi hai; profit hi profit hai,” he said, quoting a line from “Vicky Donor” that translates to “there is no investment, only profit.”

That movie did more to raise the profile of sperm donation in Bihar than anything else, said Dr. Kumar.

“The supply of sperm in my bank has increased by 25 percent after the movie ‘Vicky Donor,’ but we still need more Vicky Aroras in Bihar,” he said, referring to the leading character of the film.