The seminal plasma of eight males with normospermia, seven with oligozoospermia, and eight with azoospermia was examined for the presence of IgG and IgM by means of double-gel immunodiffusion. IgG was present in every seminal plasma sample. The highest titers of IgG were found in azoospermic and normospermic samples, but the lowest IgG titers were seen only in normospermic samples. IgM was detected only in some cases, mainly in the azoospermic group. Testicular biopsy was performed in all the azoospermic patients, but IgM was found only in the cases demonstrating spermatogenic arrest and was not found in the cases with complete spermatogenesis. It is concluded that the IgM present in the seminal plasma was of testicular origin and was associated with injury of the male genital system.