An animal model was used to examine the effect of human colostrum on an acute inflammatory process in vivo. Subcutaneous air pouches on the backs of outbred rats were injected with carrageenan as an inflammatory challenge, normal saline, pooled aqueous human colostrum, or carrageenan plus colostrum concurrently. Oral dexamethasone or indomethacin was administered to some animals before and during challenge as anti-inflammatory agents. Polymorphonuclear leukocyte (PMN) counts in pouch fluid were determined 6 h postchallenge. Carrageenan challenge resulted in a significant acute inflammatory response [48.8 +/- 4.9 x 10(6) PMN/pouch (mean +/- SEM, n = 46)] compared with normal saline controls [0.9 +/- 0.2 x 10(6) (n = 31, p < 0.001 versus carrageenan)] or with colostrum [4.3 +/- 0.8 x 10(6) PMN/pouch (n = 11, p < 0.001 versus carrageenan)]. The concurrent injection of colostrum plus carrageenan challenge significantly reduced the PMN response compared with carrageenan alone [18.8 +/- 2.9 versus 48.8 +/- 4.9 x 10(6) PMN/pouch (carrageenan plus colostrum versus carrageenan, n = 41 versus 46, p < 0.001)]. This degree of suppression of PMN influx was not significantly different from that seen with indomethacin treatment but was significantly more than that seen with dexamethasone treatment. The decreases in PMN counts observed most likely reflect suppression of the acute inflammatory response because a significant amount of PMN lysis in colostrum was not observed in vitro and the accumulation of PMN granule contents was not seen in pouch fluid from colostrum-treated animals in vivo. These data directly demonstrate for the first time that human colostrum has a biologically significant effect on the inflammatory process in vivo.