Hormone-like substances were isolated from culture media of haploid strains of Saccharomyces cerevisiae. The one excreted by cells of mating type a made cells of the α type expand; the other, excreted by α type cells, made cells of the a type expand. Tentatively we call the former a hormone and the latter α hormone. The cell-expanding action of the a hormone was inhibited by actinomycin D, chloramphenicol and cycloheximide. The a hormone was shown to be heat-stable and dialyzable. Both hormones could be extracted with methylene chloride. The abilities of cells to produce these hormones and to respond to them are under control by the mating-type genes.