Previous workers have suggested that Streptococcus pneumoniae, the pneumococcus, produces multiple forms of the enzyme neuraminidase. By serial chromatography on DEAE-cellulose, Sephacryl S-200, Amicon Red-A gel and hydroxylapatite we have purified to electrophoretic homogeneity a pneumococcal neuraminidase with an apparent molecular weight of 86,000 (as determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis). Mouse antiserum raised against the purified material reacted with a single species with molecular weight 107,000 (107K form) in crude pneumococcal cell lysate. During the purification procedure this species was progressively degraded to the molecular weight 86,000 (86K) form whilst retaining enzyme activity. Degradation of neuraminidase was inhibited by phenylmethylsulphonylfluoride (PMSF) and ethylenediaminetetraacetic acid (EDTA). Purification of the enzyme in the presence of these protease inhibitors permitted the isolation of the 107K species substantially undegraded and greater than 98% pure. Our findings on the degradation of neuraminidase during its purification account for previous reports of multiple neuraminidase isoenzymes in Streptococcus pneumoniae.