This study investigated the in vitro toxic effects of Bitis arietans venom and the ability of antivenom produced by the South African Institute of Medical Research (SAIMR) to neutralize these effects. The venom (50 microg/mL) reduced nerve-mediated twitches of the chick biventer muscle to 19 +/- 2 of initial magnitude (n = 4) within 2 h. This inhibitory effect of the venom was significantly attenuated by prior incubation of tissues with SAIMR antivenom (0.864 microg/microL; 67 +/- 4 ; P <0.05; n = 3-5, unpaired t-test). Addition of antivenom at t50 failed to prevent further inhibition or reverse the inhibition of twitches and responses to agonists. The myotoxic action of the venom (50 microg/mL) was evidenced by a decrease in direct twitches (30 +/- 6 of the initial twitch magnitude) and increase in baseline tension (by 0.7 +/- 0.3 g within 3 h) of the chick biventer. Antivenom failed to block these effects. Antivenom however prevented the venom induced cytotoxic effects on L6 skeletal muscle cells. Venom induced a marginal but significant reduction in plasma clotting times at concentrations above 7.8 microg/100 microL of plasma, indicating poor procoagulant effects. In addition, the results of western immunoblotting indicate strong immunoreactivity with venom proteins, thus warranting further detailed studies on the neutralization of the effects of individual venom toxins by antivenom.