Our own studies on the yeast killer phenomenon have been concentrated on its application for the differentiation of opportunistic pathogenic yeast isolates within the same species and its use as an epidemiological marker in nosocomial infections caused by yeasts. Our most recent investigations have led us to reevaluate the potential uses of this phenomenon, since it is now apparent that other microorganisms, unrelated to yeasts, are susceptible to the effects of these toxins. The yeast killer phenomenon can theoretically be used to study epidemiological aspects of any pathogenic microorganism, especially when other systems are not available. Monoclonal antibodies produced against a crude toxic extract of a killer yeast (Pichia anomala UCSC 25F) active against a large number of microorganisms were used to carry out a serological study on metabolic products of various yeasts with known and unknown genetic determinants of their killer characteristics. The extract itself had demonstrated a therapeutic effect in vivo when applied topically. Anti-idiotypic antibodies against these monoclonal antibodies were raised in rabbits. In vitro, these anti-Ids mimicked the action of the killer toxin used as immunogen in the production of monoclonal antibodies. The perspectives of investigations on yeast killer phenomenon are discussed.