Production of erythrogenic toxin type B by Streptococcus pyogenes strain T19 was found to be strongly dependent on the pH of the cultivation medium. Maximum yields (greater than 100 mg of toxin/1) were obtained at pH 6.0. In contrast no toxin production was serologically detectable at pH values above 6.5. Purified B-toxin was shown to consist of two components when assayed by SDS-electrophoresis. The molecular weight of the two components was estimated to be 30 000 and 12 000. Isoelectric focusing revealed a heterogeneity of the preparation with isoelectric points between 8.0 and 9.0. Streptococcal proteinase precursor was isolated from culture supernatants of strains T19 and B220 by ammonium sulfate crystallization and purification on CM-Sepharose CL 6B. The protein obtained was homogeneous by SDS-gel electrophoresis and had a molecular weight of 44 000. After autocatalytic activation with mercaptoethanol two bands appeared corresponding to molecular weights 30 000 and 12 000. Isoelectric focusing of proteinase precursor preparations yielded a double band at pI 8.2-8.3. However, activation of precursor to active proteinase finally resulted in a change of the pI to 9.0. Erythrogenic toxin type B, streptococcal proteinase precursor, its intermediate activation products and the active proteinase itself reacted serologically identical with anti B-toxin antiserum. Streptococcal proteinase precursor provoked a delayed skin reaction and was pyrogenic as well as mitogenic. Its pyrogenic activity could be inhibited by antiserum against scarlet fever toxin (Wellcome Laboratories). We therefore believe erythrogenic toxin type B to be identical with streptococcal proteinase precursor. This helps to understand the heterogeneity of B toxin, its inactivation by trypsin and the different protocols for toxin production described in the literature.