Spontaneous, low-frequency loss of ability to produce streptococcin A-FF22 (SA) by group A streptococcus strain FF22 was observed. The proportion of non-SA-producing (SA(-)) derivatives occurring in strain FF22 cultures grown in Todd Hewitt broth supplemented with 1% of yeast extract (THBY) was increased on treatment with ethidium bromide, acriflavin, or rifampin. The highest incidence of SA(-) organisms, however, was found in untreated THBY cultures that had been aging by incubation at 37 degrees C for several months. The possibility of selective effects in these experiments, operating to enhance the apparent frequency of SA(-) bacteria, was discounted. The survival of SA(-) derivatives in association with populations of SA(+) bacteria was dependent upon the use of culture conditions inimical to SA activity, since a consistent finding was that the loss of ability to produce SA was associated with loss of immunity to the killing action of this bacteriocin. Whereas selective killing of SA(-) derivatives was evident in mixed cultures of SA(+) and SA(-) strains in tryptic soy broth, no such effect was demonstrable in THBY. In these experiments, elimination of SA(-) cells seemed directly related to the presence of active SA. Purified clones of SA(-) substrains did not seem revertible to SA production, either spontaneously or on treatment with nitrosoguanidine. It is suggested that the property of production of SA by group A streptococcus strain FF22, together with that of host cell immunity to the homologous bacteriocin, may be mediated by plasmid-borne genetic determinants.