In Kluyveromyces (Saccharomyces) lactis, stocks of complementary mating type each produce a diffusible agent that specifically affects the cells of the opposite sex. The active agents influence bud orientation and growth in such a way that the cells of opposite mating type bud toward each other. The response occurs when the cells are grown on malt extract (ME) medium only; the reaction is not observed during growth on a yeast malt (YM) medium.