Based on the selective toxicity of yeast killer toxins towards other yeasts, a search was mounted for pharmacologically active proteins effective against Candida albicans and Candida glabrata. the principal aetiologlcal agents of vaginal candidosis. Of the twenty killer yeasts screened, only Williopsls mrakii LKB169 and NCYC500 displayed strong anti-Candida activity and were selected for further study. Unfortunately both yeasts proved unstable following serial subculture on agar slants. A secretion mutant derived from LKB169 (denoted LKB169A) lacked antimicrobial activity against both _C albicans and Saccharomyces cerevislae but retained activity against £ glabrata. A working hypothesis proposing the existence of multiple toxins from W mrakii was further investigated. A triplet bioassay using _C albicans A72, _C glabrata NCYC388 and cerevislae NCYC1006 was developed to differentiate toxins from W mrakii. Based on differences in thermal decay, sensitivity to papain and specificity of action, three distinct protein toxins from W mrakii LKB169 were identified and designated KA, KG and RS. Toxin multiplicity was substantiated for strain NCYG500 also; production was characterized by an early loss of antimicrobial activity against C albicans with a concurrent reversal in the ratio of the activity against C glabrata to the activity against S cerevislae. In general toxins from strain NCYC500 were more stable than those from LKB169. pH was identified as an important factor in the thermostability of Williopsls toxins. For example, the KG toxin from W mrakii LKB169 retained full activity against C glabrata after boiling for 10 minutes at pH 4.0; however, at pH 4.5 biological activity decayed at 37°C. Williopsls toxins may have therapeutic potential in vaginal candidosis where the low pH favours activity. In this regard it is encouraging that a secreted acid proteinase from C albicans did not hydrolpse the Williopsls toxins jLn vitro.