Both pyran copolymer and the Burroughs-Wellcome vaccine of Corynebacterium parvum were able to elicit a migratory response by human neutrophils or monocytes when incubated with serum. However, in the absence of serum, neither agent elicited such a response. The generation of the chemotactic factor by pyran was dependent on heat-labile components in the serum, while suggested involvement of the complement sequence. Conversely, the migratory response of C. parvum in heated serum was equivalent to that in normal serum. No specificity for either monocytes or neutrophils was observed.