Considerable amounts (200 units/ml) of neuraminidase activity were detected in middle ear effusion of children (age 1 month-10 years) and its presence was highly correlated with the presence of Streptococcus pneumoniae. When isolates of this organism are cultured, neuraminidase activity appears in the growth medium during the exponential phase of growth. In order to study the role of this enzyme in the pathology of otitis media we have developed a method for its purification. The enzyme was purified over 5,800-fold by removing the organism and passing the culture broth through a series of affinity and ion-exchange columns. The overall yield was 2 mg enzyme protein and the final specific activity was 1.8 X 10(6) units/mg protein. A molecular weight of 65,000 was estimated by SDS-PAGE and gel filtration chromatography. The Stokes radius of neuraminidase was calculated to be 32 A, its isoelectric point was 7.2, and its pH optimum was 6.0. In terms of specificity, the enzyme catalyzed the hydrolysis of sialic acid linkages in mucin, glycoproteins, and gangliosides: bovine submaxillary mucin supported the highest catalytic efficiency, and alpha-1-antitrypsin the lowest. Neuraminidase acted on at least three linkage classes of substrates, alpha-2,6 and alpha-2,3 linkages of N-acetylneuraminic acid to galactose, and alpha-2,6 linkages to N-acetyl-galactosamine.