The purpose of this study was to examine some of the pharmacological actions of venom from the Australian bulldog ant Myrmecia pyriformis. M. pyriformis venom was prepared by extraction of venom sacs in distilled water and centrifugation to remove insoluble material. Venom (2 micrograms/ml) produced a biphasic response of isolated guinea-pig ileum, i.e., an initial rapid contraction followed by a slower prolonged contraction. The histamine antagonist mepyramine (0.1 microM) inhibited the first phase of this response by approximately 80%. In the isolated rat stomach fundus strip (histamine-insensitive), venom (2-4 micrograms/ml) produced only a single contraction. Responses to venom of egg-albumin-sensitized guinea-pig ileum, were not significantly different before and after an anaphylactic response induced in vitro by egg albumin (0.5 mg/ml). Fluorometric assay showed that histamine accounted for 3.5 +/- 0.5% of the dry weight of M. pyriformis venom. Both the lipoxygenase/cyclooxygenase inhibitor BW755C and the cyclooxygenase inhibitor indomethacin significantly inhibited the response to venom of guinea-pig ileum (second phase) and rat fundus strip. M. pyriformis venom caused hemolysis of guinea pig blood. The degree of hemolysis was reduced significantly when boiled venom was used. No evidence was found that the venom contains acetylcholine, bradykinin, or 5-hydroxytryptamine or that the venom releases histamine from guinea-pig ileum. However, the results indicate that the venom contains histamine-like activity. In addition the venom was found to cause the release of cyclooxygenase products and to contain a heat-sensitive hemolytic factor.