Agglutinins in S. cerevisiae are necessary for mating, for recognition between cells of opposite mating type. The mode of agglutinin synthesis is altered by the growth temperature and by the carbon source, from constitutive to inducible synthesis and vice versa. Some of the unsaturated fatty acids tested induced synthesis of agglutinins in cells grown at an elevated temperature, even in the absence of the pheromone. However, synthesis of agglutinins in glycerol-grown cells, that are inducible by the pheromone, was not induced by linolenic acid. Hence, the change in the mechanism of regulation of agglutinability produced by differences in temperature probably differs from that produced by differences in carbon source.