Despite a 90% cryosurvival of Trichomonas vaginalis in their growth medium trypticase yeast maltose (TYM) with DMSO, none of these parasites have previously been observed to survive during cryopreservation of infected human semen with glycerol (Andrologia 18, 323 (1986)). This could have been due to the failure of the culture method used to detect low numbers of survivors. The prospects of possible transmission of T. vaginalis by artificial insemination with cryobanked (-196 degrees C) semen prompted an investigation of the cryosurvival of this parasite in the presence of semen with the cryoprotectant glycerol, using a more sensitive culture method for viability evaluation. Semen and seminal fluid from the same 23 ejaculates, as well as culture medium, were inoculated with small clinical numbers of T. vaginalis and evaluated as to their survival before and after cryopreservation. Results indicated: (i) The highest cryosurvival of T. vaginalis (4.5%) was in cryobanked (glycerolated) semen, (ii) semen, as well as glycerol, shows cryoprotective action, and (iii) glycerol reduced survival of parasites in semen, seminal fluid, and TYM medium during exposure prior to freezing. Clinical information on infectivity of small numbers of T. vaginalis and the data presented here suggests that these organisms could be transmitted by artificial insemination with infected cryobanked human semen.