The galactosyltransferase activity was determined in seminal plasma from 24 men with various sperm characteristics subdivided in three different groups and from 9 men subjected to vasectomy for the purpose of voluntary sterilization contributing a 4th group. The galactosyltransferase displayed an obligatory dependency on manganese ions for activity. Endogenous glycoproteins of human seminal plasma were poor acceptors for the galactosyltransferase reaction, and a more than 40-fold increase in activity was achieved in the presence of ovomucoid as exogenous acceptor. There were no significant intergroup differences with regard to galactosyltransferase activity. Hence, qualitative and quantitative discrepancies of spermatozoa were not influential on galactosyltransferase activity in a decisive way. An overall positive correlation was registered between galactosyltransferase activity and sperm penetration in cervical mucus (r = 0.53; p less than 0.01).