THE HAGUE — Twelve people who were conceived with sperm from a Dutch fertility center have filed a lawsuit asserting that its longtime director is their biological father, and that over several decades, he swapped donors’ sperm with his own.

The 12 people, and 10 mothers who suspect that their children were conceived using the clinic director’s sperm, filed a lawsuit on Friday asking a court in Rotterdam to give them access to the DNA of the clinic director, Dr. Jan Karbaat, who died last month at 89.

“I’m hoping that the judge will allow us to extract the DNA so we can use it to find out if we are his children,” one plaintiff, Moniek Wassenaar, 36, said in an interview. The 12 people are 8 to 36 years old. Some of the 10 mothers in the suit conceived children who are still minors.

From 1980 to 2009, Dr. Karbaat ran a sperm bank in the rear of his stately yellow brick house in the Bijdorp section of Barendrecht, near Rotterdam. He became well known in the field of assisted reproduction. About 10,000 children are estimated to have been conceived at the clinic.