The alpha (α) component of the c protein and R proteins are trypsin resistant, but antigenically distinct, proteins on the cell surface of some but not all strains of group B streptococci (GBs). These two classes of proteins, along with the group and type polysaccharide, can be used to characterize strains of GBS. Four species of R protein (R1 through R4) have been described. We studied trypsin extracts from numerous strains of GBS by immunodiffusion in agarose and polyacrylamide gel electrophoresis/Western blot. Sera monospecific for α, R1 and R4 were used to immunoprecipitate/blot the proteins. The molecular weight of the blotted proteins was determined. Although by immunodiffusion the proteins within a class were identical to each other, great heterogeneity in size and blotting pattern was found within each class. Variation was independent of the polysaccharide serotype. Multiple molecular weight species were seen for α, R1 and R4 proteins. For a given strain, the various forms of α or R1 appeared to form a multiple size ladder; those of R4 were fewer and closer in size. The highest form of α ranged from 85 to 170 kDa, with 45 kDa being the highest form for some rare GBS strains. For R4 the predominant and highest form varied from 84 to 197 kDa, whereas some strains with R1 had the highest form over 200 kDa. Our results indicated that despite similarities, there is great diversity among the α, R1 and R4 trypsin resistant proteins of GBS.