Lymphokine (LK) preparations, containing macrophage aggregating activity, also aggregated purified neutrophils, but not blood mononuclear cells or erythrocytes. Additionally, aggregation of blood leucocytes (containing 50%-70% neutrophils), from sensitized guinea-pigs, could be induced by culture of these cells with antigen, in a direct aggregation assay. Optimal measurement of direct aggregation was at 6 hr, whereas soluble neutrophil aggregating activity was produced by blood mononuclear cell cultures continuously for at least 24 hr. Sephadex fractionation showed that although macrophage and neutrophil aggregating activity was associated with molecules of mean mol. wt 50,000, neutrophils were also aggregated by material in the 90,000-225,000 mol. wt range. LK-induced neutrophil aggregation, like macrophage aggregation, was inhibited by colchicine, prostaglandin E2, chlorpromazine and the glycolytic inhibitors 2-deoxyglucose and iodoacetate.