Pneumococcal surface protein A (PspA) has been shown to be a virulence factor of pneumococci and to elicit protective anti-pneumococcal antibodies in mice. PspAs from different pneumococcal isolates have been shown to exhibit antigenic variability. In previous studies with three strains, two different apparent molecular weights of PspA were observed. In this report we have studied the variation in molecular weight of PspA from 43 pneumococcal strains reactive with anti-PspA monoclonal antibodies, Xi64 and/or Xi126. The relative molecular mass (Mr) of the major PspA band ranged from 67 k to 99 k in the different strains. Variations in Mr of PspA were observed even within strains of the same capsular type. The molecular size of PspA from strain Rx1 was not affected by treatment with a variety of chemical, enzymatic, and physical procedures, suggesting that the differences in Mr of PspA among different strains, was not due to uncontrolled variations in PspA preparation. The Mr of PspA of a given strain was found to be stable both in vivo and in vitro. As a result variations in the Mr of PspA from clinical isolates, should allow discrimination between strains within a given capsular type in epidemiologic studies. © 1990.