SummaryNeonatal polymorphonuclear leukocytes (PMNs) have previously been shown to be chemotactically deficient. To probe the mechanism(s) responsible for this deficiency, we have investigated the phenomenon of concanavalin A-induced capping in neonatal PMNs. PMNs from cord blood of 17 healthy, full-term infants and 17 normal adult volunteers were isolated by standard Ficoll-Hypaque and dextran sedimentation. After incubation with and without colchicine, the cells were reincubated with fluorescein isothiocyanate-Con A, fixed, and prepared in wet mounts. Using a fluorescence microscope, PMNs were identified, and the percentage of the capped cells was counted.Upon treatment with colchicine, adult PMNs showed a significant increase in the percentage of capped cells. By contrast, the cord blood PMNs showed no significant increase in capping after colchicine treatment. The difference between percentage of PMNs showing colchicine-induced capping in adult and cord blood was highly significant (P< 0.01; Student'sttest).