A heterologous rabbit anti-rat neutrophil serum (ANS) based on peptone-stimulated peritoneal exudate neutrophils (PMNLs) from Sprague Dawley rats was prepared. Leucoagglutination and indirect immunofluorescence assays revealed high titres of antibodies to rat PMNLs (1/2560), lower titre of antibodies to rat lymphocytes (1/160) and a very low titre against rat platelets (1/20). ANS given intravenously (i.v.) to rats in doses of up to 42 mg of protein/kg b.w. caused transient neutropenia, lasting about 10 min after administration, and thrombocytopenia, lasting about 5 min. Two minutes after an i.v. injection of 21 mg of ANS/kg b.w. there was profound uptake of 51Cr-labelled PMNLs in the lung, increased release of 51Cr to plasma, an increased amount of 51Cr in the spleen and consumption of greater than 98% of total complement (CH50). Two hours later there was high activity of 51Cr in the plasma, spleen and liver, while lung radioactivity had decreased to below baseline and CH50 had recovered to 55% of baseline. An intraperitoneal (i.p.) injection of ANS was followed by prolonged neutropenia with a maximum after 12 h. Simultaneously peripheral mononuclear cells slightly decreased. There was no change in the number of peripheral platelets in the blood or in the plasma concentration of fibrinogen, alpha 2-antiplasmin, plasminogen or plasminogen activators. Intraperitoneal administration of ANS did not affect CH50. It was concluded that the raised ANS had good specificity against rat PMNLs and was able to induce prolonged neutropenia after i.p. injection without affecting the complement of fibrinolytic system.