Phytohemmagglutinin (PHA)-induced lectin aggregation, chemotactic response to C5a, and random migration were measured on paired samples of neutrophils obtained from human peripheral blood and cord blood of normal newborn infants. The mean aggregation rate (+/- 1 SD) of adult neutrophils with PHA was 16.8 +/- 4.4 vs. 12.0 +/- 3.6 for newborn neutrophils (P less than 0.005), and the mean percent aggregation of adult neutrophils was 56.2 +/- 9.2 vs. 45.6 +/- 8.3 for newborn neutrophils (P less than 0.005). Exposure to newborn plasma had no affect on adult neutrophil aggregation. Whereas vinblastine (VBL) decreased both the percent and rate of PHA-induced adult neutrophil aggregation, only the rate of newborn neutrophil aggregation was reduced by VBL. Newborn neutrophil chemotactic response to C5a, was reduced by 80% (P less than 0.0025), and showed a positive correlation with percent PHA-induced aggregation (r = 0.6037, P less than 0.05). On the other hand, random migration was not significantly reduced and did not correlate with PHA-induced aggregation. These observations suggest that the decreased chemotactic responsiveness of newborn neutrophils may be due to developmental membrane differences which adversely affect the number and/or availability of C5a resceptors. Lectin-induced aggregation studies of other chemotactic defects may identify similar differences.