Dehydroabietic acid (DHAA) is a major aquatic toxic resin acid usually found in unbleached pulp mill effluents. This compound has been reported to accumulate in the red cells of rainbow trout and to cause hemolysis. To elucidate further understanding to the mechanism of action of this resin, the interaction of DHAA with human erythrocyte membranes has been monitored by electron paramagnetic resonance techniques of spin labeling. Results presented in this paper indicate that DHAA, in a concentration-dependent manner, significantly altered both the motion and order of the lipid bilayer and the physical state of cytoskeletal proteins, while DHAA had no effect on isolated lipids. It is proposed that the increase in the 'fluidity' of the lipid bilayer induced by DHAA is a secondary effect of primary changes in the physical state of the cytoskeletal proteins of the membrane, and that the latter effect is critically associated with the toxicity of DHAA.