Granulocyte transfusions (GT), 0.98 X 10(9) neutrophils/kg of body weight, were performed on 7 healthy pony foals between 2 and 7 days old. The mean neutrophil count of the foals was significantly (P less than 0.05) greater than base line (4,830 +/- 1,260/microliter) 1 hour after GT (8,870 +/- 3,350/microliter) and was similar to base line by 15 to 18 hours after GT (6,550 +/- 2,310/microliter). Leukocyte concentrates (LC) used for GT were harvested from clinically normal adult horses by continuous-flow centrifugation leukapheresis (CL), 3 to 6 hours after hydrocortisone sodium succinate was administered to increase the blood neutrophil count. The mean neutrophil count of the LC used for GT was 68,050 +/- 13,990/microliter, and the mean LC volume was 377.4 +/- 79.2 ml (14.82 +/- 3.54 ml/kg). The mean time required to collect the LC used for GT was 232.1 +/- 73.4 minutes. Neutrophils from LC had significantly reduced in vitro stimulated migration to zymosan-activated serum, when compared with peripheral blood neutrophils of the donors (P less than 0.05). Neutrophil phagocytosis and bactericidal capacity were not significantly changed in LC. Mean neutrophil migration indices were not significantly different in foals after GT. Mild depression and transient diarrhea was noticed in 1 foal 30 minutes after the start of the GT. The donor of LC for this foal and 1 other donor experienced depression, piloerection, and muscle tremors during CL, indicating that complement had been activated. Problems were eliminated by the use of new disposable plastic materials for blood processing in each CL procedure.