The systemic secretion of a serine protease, rat mast-cell protease II (RMCPII), a major product of rat mucosal mast cells (MMC), was measured by a sensitive enzyme-linked immunosorbent assay (ELISA) in the sera of naive and primed rats challenged with the nematode N. brasiliensis. The systemic secretion of RMCPII was both time- and dose-dependent in primed rats (1-3 micrograms RMCPII/ml serum, 1 hr after challenge). Systemic release of RMCPII was slower and less pronounced in naive rats following intraduodenal challenge with 5-day-old worms. No RMCPII was detected in the sera of naive rats challenged with 4-day-old N. brasiliensis. Soluble worm antigen had no effect in naive rats, but when it was given intraduodenally or intravenously to primed rats, the serum levels of RMCPII 1 hr later were 10.5 micrograms/ml and 122 micrograms/ml, respectively. Few morphological changes were detected in MMC following worm challenge and the jejunal content of RMCPII was unaltered. A substantial reduction in the number of MMC occurred following intravenous injection of worm antigen, and the remaining cells were vacuolated and pale-staining, although granule exocytosis was not observed. Significant reduction in the jejunal content of RMCPII was also evident. These results demonstrate, unequivocally, that MMC are activated in response to N. brasiliensis challenge infection or to parasite antigens. In addition, the ability to detect secreted RMCPII in the sera of test animals provides a highly sensitive and uniquely selective assay to determine the participation of MMC in pathological reactions at mucosal surfaces.