Pleiotropic alteration of several genetic characters including toxin production was quantitatively shown with a strain of Staphylococcus aureus of phage type 80, 81 which had been given a very specific genetic marker (temperature sensitivity of mannitol fermentation) to avoid confusion by contamination. Thus, alpha-hemolysin hyperproducers obtained by N-methyl-N'-nitro-N-nitrosoguanidine (NTG) mutagenesis were very often hyperproducers of DNase, coagulase, and protease. Their colonies were less yellow than the parent. DNase hyperproducers obtained after NTG mutagenesis were also often hyperproducers of alpha-hemolysin, coagulase, and protease, with colonies less yellow than the parent. Almost all of the revertants obtained by mutagenesis with ethyl methane sulfonate with respect to alpha-hemolysin or DNase were shown to have simultaneously become hypoproducers of alpha-hemolysin, DNase, and protease. Since the pleiotropic alteration of multiple functions was thus quantitatively confirmed, the mechanism underlying this phenomenon should probably be related to a regulatory mechanism common to them.