Certain extracts of streptococcal cell walls are known to inhibit macrophage migration in vitro. In this study, we attempted to identify the streptococcal components responsible for this phenomenon. Trypsinized cell walls and cytoplasm from groups A and B streptococci were extracted with hot formamide followed by acetone precipitation. Subsequent gel filtration in aqueous solutions yielded a fraction devoid of C-carbohydrate and containing mostly oligonucleotides, apparently derived from streptococcal cytoplasm. This fraction significantly inhibited the migration of peritoneal exudate cells from rats sensitized to groups A and B streptococci. It was noteworthy that no inhibition of migration was observed with cells from nonsensitized animals or control rats injected with BCG or complete Freund adjuvant. Similarly, no inhibition was obtained with formamide extracts of calf thymus RNA. Although the inhibition does not show specificity for streptococcal groups, it seems to have immunological specificity since prior sensitization with streptococci is required for migration inhibition.