When 30 clinical isolates of Streptococcus pneumoniae, representing 16 capsular serotypes, were analysed by Western blot for production of the haemolytic toxin pneumolysin (Ply), all strains produced an immunoreactive band of similar intensity. However, six isolates of serotype 8 and two of type 7F expressed Ply whose mobility on SDS-PAGE was anomalously slow. Culture lysates from these strains also had low haemolytic activities compared with those for clinical isolates of other serotypes, suggesting the possibility of mutations affecting specific activity. Genes encoding Ply from one type 8 isolate and one type 7F isolate were cloned into Escherichia coli and sequenced. Compared with the published sequence for Ply, the deduced amino acid sequence for the type 8 Ply variant contained three amino acid substitutions, and the type 7F variant four amino acid substitutions. Both variants also had Val270 and Lys271 deleted. The variant Ply proteins were purified from recombinant E. coli expressing the cloned genes, and shown to have substantially reduced specific haemolytic activities [6.8 x 10(4) haemolytic units (HU)/mg and 2.3 x 10(4) HU/mg for type 8 Ply and type 7F Ply respectively] compared with Ply itself (1.2 x 10(6) HU/mg). Studies with chimeric toxin gene constructs indicated that both the reduced haemolytic specific activity and the anomalous electrophoretic mobility of the variant Plys were attributable to a single amino acid substitution (Thr172-->Ile).