The determination of tissue plasminogen activator (t-PA) antigen in semen by a new immunoenzymatic method was done in 152 men without genito urinary pathology. The seminal value is one hundred higher than the blood value with a log normal distribution. These levels are not influenced by freezing temperature, by the sequences freezing-thawing, by centrifugation. From split ejaculates a correlation has been established between t-PA antigen and zinc and not with fructose. These results indicate the major prostatic origin of the t-PA antigen.