It has been suggested that high zinc concentrations found in men may prevent Trichomonas vaginalis from being established in the male reproductive tract. In this investigation T vaginalis was readily killed at concentrations of zinc that occur in the prostatic fluid of healthy men (minimum trichomonacidal concentration (MTC) of 6.4 mmol/l). T vaginalis was also shown to be killed by human prostatic extracts as well as by human seminal fluid, even when the zinc content was much lower than the MTC for T vaginalis. It seems likely, therefore, that there are at least two antitrichomonal mechanisms in the male reproductive tract, one being zinc dependent and the other not relating to zinc content. Tritrichomonas foetus, which causes venereal trichomoniasis in cattle, was unaffected by bovine seminal fluid and was killed by zinc only at concentrations far higher than those found in the prostatic fluid in the bull (MTC 200 mmol/l).