Summary.Twenty infertile patients with normal tubal patency were inseminated intraperitoneally (11 once, seven twice, and two three times) with spermatozoa (mean 14 times 106, range 0.6–48 times 106) prepared by the standard swim‐up technique. The occurrence of immunization to spermatozoa was looked for by the Gelatin Agglutination Test (GAT) and Tray Agglutination Test (TAT). Both tests gave negative results for all the controls (10 pregnant and 10 puerperal women).Antisperm antibodies were measured in the serum before, 30 d and 4–7 months after Intraperitoneal Insemination (IPI).The last check was done for only 14 patients, since six became pregnant as a consequence of the first treatment. Of the 14 patients studied after 4–7 months, seven had two, and two had three IPI.In the group of inseminated patients, 18 women with no basal sperm antibody did not show evidence of antibody formation after the treatment and it was not increased after insemination in the two patients who already had low antibody titre (1/32).In conclusion, despite the large number of spermatozoa inseminated and even after several IPI attempts, there was no evidence ofde novoproduction of or increase in already present anti‐sperm antibodies according to the methods used for the detection of ASA in t