Immunogel-diffusion studies showed that 60°C LiCl extracts of the smooth serum-resistant mutant strain 17 contained greater amounts of a protein antigen than did extracts of the parent strain LP729. An extract of strain 17 was fractionated on Sepharose 4B and the protein antigen was found as the only detectable antigen in a number of fractions; sodium dodecyl sulphate-polyacrylamide gel electrophoresis indicated that these fractions contained one major polypeptide band with a molecular weight of 46 000 daltons. We suggest that this protein antigen may be partly responsible for the serum resistance of strain 17 though its presence in other serum-sensitive strains suggests that additional factors are essential for full serum resistance.


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