5(S), 12(S)-Dihydroxy-cis-14,trans-6,8,10-eicosatetraenoate (compound I), 5(S),12(R)-dihydroxy-cis-14,trans-6,8,10-eicosatetraenoate (compound II), and 5(S),12(R)-dihydroxy-cis-6,14,trans-8,10-eicosatetraenoate (compound III) were prepared from rabbit peritoneal neutrophils challenged with arachidonic acid plus ionophore A23187. Each arachidonate metabolite caused rabbit neutrophils to aggregate and, in cells treated with cytochalasin B, release granule-bound enzymes. Compound III was 10- to 100-fold more potent than compounds II and I. When intravenously infused into rabbits at doses of 100--1,000 ng/kg, compound III induced abrupt, profound, transient neutropenia associated with a rapidly reversing accumulation of neutrophils in the pulmonary circulation. This in vivo action correlated closely with the ability of the fatty acid to activate neutrophils in vitro: neutropenia, aggregation, and degranulation occurred at similar doses of stimulus and the rapid, reversing kinetics of the neutropenic response paralleled the equally rapid, reversing formation of aggregates. The fatty acid did not alter the circulating levels of lymphocytes or platelets and did not aggregate platelets in vitro. At comparable doses (i.e., 100--1,000 ng/kg), compounds I and II did not cause neutropenia. Thus, compound III possesses a high degree of structural and target-cell specificity in stimulating neutrophils in vitro and in vivo. Clinical and experimental syndromes associating neutropenia with increased levels of circulating arachidonate metabolites may involve compound III as a mediator of neutrophil sequestration in lung.