The effects on normal polymorphonuclear leukocytes (PMNs) of chemotactic factors in sera incubated with five types of S. pneumoniae commonly encountered in otitis media and other respiratory infections (types 3, 6A, 14, 19F and 23F) and five types that rarely cause infections (types 31, 35, 36, 42 and 43) were compared by the means of migration under agarose technique. All the types frequent in otitis media and some of the types that are less frequent, viz. 31 and 36, showed a good chemotactic activation in pooled serum. No correlation between tendency to cause otitis media and ability to regenerate chemotactic activity in serum was found. Impaired generation of chemotactic activity was found in C2-deficient serum, Mg2 + EGTA-chelated serum and properdin-deficient serum, indicating the significance of both the classical and the alternative complement pathways for production of chemotactic factors by pneumococcal types.