Ca 19-9 levels were determined by radioimmunoassay in the seminal plasma of 108 men divided in (a) eight groups according to etiological diagnosis of infertility and (b) two groups on the basis of normal or abnormal spermiogram. The mean Ca 19-9 (or Ca 19-9-like activity) in the seminal plasma was 114.6 times higher than the upper limit in normal serum. A statistically significant difference was noted in the antigen levels between men with normal and those with abnormal spermiograms, although with wide overlapping of the individual values. There was no correlation between Ca 19-9 levels and the variables of the spermiogram. The prostate seems to be the main site of origin of Ca 19-9 or Ca 19-9-like activity in the seminal plasma.