RNA-core (RNAase-resistant fraction of yeast RNA) induced streptolysin S (SLS) was purified (40% recovery) to apparent electrophoretic homogeneity by hydroxylapatite chromatography followed by gel filtration on Sephadex G-100 in the presence of 6 M-guanidine. HCl. The specific activity of the purified toxin was 3 X 10(6) haemolytic units (mg protein)-1. The Mr of the toxin was below 4000 on the basis of SDS-PAGE and 20 000 by gel filtration in guanidine. HCl. High-voltage isoelectric focusing of the purified toxin allowed the isolation of the carrier-free SLS peptide for the first time. This peptide was basic (pI 9.2) as compared to native SLS (pI 3.6). The native toxin and the peptide had similar haemolytic properties except for the high lability of the peptide, which was stabilized by RNA-core. The Mr of the denatured peptide was about 1800, as estimated by gel filtration.

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