Neutrophil (PMN) counts, immune complex (IC) uptake by PMN, and serum opsonising activity for promoting yeast uptake were used to evaluate infection clearing capacity in 16 lambs prior to colostrum feeding (two lambs fed bovine colostrum, 14 suckled lambs) and at 2 days of age. At 2 days of age lambs had more circulating PMN than they had prior to colostrum uptake (P less than 0.01). Colostrum feeding caused a significant increase in the percent of lamb PMN phagocytosing IC, although at Day 2 the percent phagocytosis was significantly lower (32.2%) than for adult controls (90%). Yeast opsonophagocytosis was greater when 24-36 h post-feeding serum was the source of opsonin than when pre-feeding serum was used (P less than 0.001). When adult serum was the opsonin, yeast opsonophagocytosis was approximately twice the phagocytosis mediated by 24-36 h post-feeding serum. The peripheral neutrocytosis and the enhancement of opsonophagocytosis generated by absorption of either ovine or bovine colostrum did not differ. The results of this study suggest that the parameters evaluated may be used for indicating the presence (or absence) of passively acquired protective immunity.