A solid phase immunoradiometric assay was developed for the quantitation of tryptase released from activated human mast cells. Tryptase exhibits a linear dose-response curve over the standard range of 2-50 micrograms/l in buffer, serum, and plasma. The dose-response curve approached a plateau at a tryptase concentration of 100 micrograms/l and exhibited partial inhibition at concentrations above 10,000 micrograms/l. The sensitivity of the assay was 0.2-0.4 micrograms/l, and the intra-assay and interassay coefficients of variation were below 4% at 2 micrograms/l or higher tryptase concentrations. The recovery of known amounts of purified tryptase added to serum ranged from 91 to 115%. Detection of tryptase was evaluated with several body fluids and was accurate in sera, plasma, bronchoalveolar lavage fluid, nasal lavage fluid, and saliva. The concentration of tryptase was examined in serum samples from 100 healthy controls; in each case the level was less than 2 micrograms/l. The immunoassay also was utilized to examine serum levels of tryptase after the onset of a hypotensive reaction in one patient receiving general anesthesia. A maximally elevated level of tryptase (25 micrograms/l) was detected at the first time point, 0.5 h, and elevated levels persisted to 6 h before a return to normal levels was documented at 24 h. Thus, the involvement of mast cell activation in hypotensive subjects can be ascertained by this new tryptase radioimmunoassay.