A group of men under investigation for infertility have been studied with regard to findings of aerobic and anaerobic bacteria in their ejaculates before and after medication. The group consisted of 27 men who received doxycycline concomitantly with their partners according to a fixed treatment schedule. Prior to medication 23 men delivered ejaculates which demonstrated bacterial growth. A total of 53 isolates were obtained. Following doxycycline treatment a significant number of men had a reduction in the number of isolates. The reduction was mainly confined to peptococci and propionibacteria. Of the 9 men whose partners subsequently conceived, 6 harboured peptococci and/or propionibacteria in their semen before but not after treatment. It is concluded that these results warrant more extensive studies of the possible part played by peptococci and propionibacteria in genital-tract infections in the male partners of infertile unions.