Recently, we reported that conjugal transfer of plasmid pERL1, determining i. a. erythromycin resistance (Emr), into group A streptococci could trigger the expression of anti‐phagocytic activity, adhesiveness, opacity factor and capacity to bind immunoglobulin Fc‐parts and beta2‐microglobulin. In the present study, ethidium bromide treatment of Emrtransconjugants allowed the selection of »cured«, erythromycin sensitive (Ems) mutants. This procedure did not affect the expression of the abovementioned characteristics. However, when plasmid pERL1 was again transferred to two such mutants, the »secondary«, Emrtransconjugants obtained showed lack of each of these properties. Our experiments thus demonstrated a »switch‐on« as well as a »switch‐off« effect, exerted by the same plasmid, pERL1, on some major pathogenic properties of gr