Single radial immunodiffusion has been used to evaluate immunoglobulins and secretory piece (SP) in human seminal plasma. Samples were collected from 90 healthy volunteers, 202 subjects submitted to vasectomy for contraceptive purposes, tested at various intervals after surgery, and 725 patients grouped according to selected andrological disorders. Results may be summarized as follows. In normal subjects IgG and IgA were constantly present (mean values +/- SD: 8.14 +/- 2.82 mg/dl; 1.91 +/- 1.03 mg/dl, respectively) while IgM were detected in trace amounts (from 0.7 to 3.3 mg/dl) in 10% of subjects, and negligible or absent in the remaining subjects. In vasectomized subjects IgG and IgA showed a significant increase only in the first 3 months after vasectomy, probably due to surgery. In andrological patients Ig showed an increase in cases with antisperm antibodies (A b) and in those with infections of genital tract and positive semen culture. On the basis of these findings and the secretory piece assay data the importance appears to be stressed of the local immunocompetent system at least in some andrological diseases.