Extracellular proteins produced by the four human commensal species of mutans streptococci were analysed. The organisms used were , serotypes and , serotype , serotype , and , serotypes and . They were grown in continuous culture at different generation times and pH values in media containing either glucose or fructose to determine the extent of variation in extracellular protein production that could occur for an individual strain. The results for different organisms grown under the same conditions were then compared. The total amount of protein of molecular mass ≥ 60 kDa varied considerably with the growth conditions and with the strain. Generally more protein was present at a higher pH, conditions under which the organisms also form more lipoteichoic acid. With respect to individual protein components SDS–PAGE proved better than isoelectric focusing for detecting phenotypic responses by a particular strain to environmental changes and differences between the different strains. Differences in the molecular masses of protein components were particularly pronounced in the regions designated P1 (185–200 kDa), P2 (130–155 kDa) and P3 (60–95 kDa). Every strain produced at least one component in the P1 region that cross-reacted with antiserum to the purified protein from serotype , a protein which is indistinguishable from antigens B and I/II. Two components in the P2 region were dominant in the case of and strains and showed glucosyltransferase (GTF) activity. GTF activity was also detected in the P3 region, particularly with strains.