Rapid intravenous (iv) infusion of protamine sulfate is associated with hypotension in humans. A possible mechanism for this hypotension is the release of inflammatory mediators, including histamine, from tissue mast cells lining the blood vessels. To determine whether protamine caused nonimmunologic release of histamine, histamine released from dispersed human skin mast cells exposed to protamine sulfate was measured. Skin from seven adult patients was washed, chopped into small tissue fragments, and incubated with collagenase, hyaluronidase, and DNAase. Dispersed mast cells were harvested after 12 h of short-term tissue culture, washed, and challenged with protamine sulfate. Histamine release was measured using an automated histamine analyzer and expressed as a per cent of total released histamine measured minus the spontaneous histamine release. Spontaneous histamine release averaged 6 +/- 1%. Protamine produced dose-related histamine release. At a concentration of 3 X 10(-3) M, protamine sulfate released 14 +/- 2% (P less than 0.05), which significantly differed from spontaneous release. This study demonstrates that protamine sulfate causes nonimmunologic histamine release in dispersed human skin mast cells. However, histamine release occurred only at concentrations much greater than those used in clinical practice. Thus, these data do not support the hypothesis that nonimmunologic histamine release is a likely mechanism for protamine-induced hypotension in vivo.