To elucidate further the pathomechanisms of cutaneous whealing in response to insect or sea-animal stings, commercial sources of bee and wasp venoms and of sea nettle nematocyst extracts, as well as crude bee and wasp venoms, were examined for the presence of histamine, leukotrienes (LT) C4, LTB4 (radioimmunoassay and reverse-phase high-pressure liquid chromatography), and neutrophil chemotactic activity (NCA). Histamine was present in all samples with the highest levels in the crude insect venoms. The same held for LTC4 with highest quantities in a liquid commercial bee venom extract and in the crude venoms. Relatively large amounts of LTB4 were recovered from the sea nettles with a correspondingly high NCA. Although small levels of LTB4 were found in the other samples, there was no clear correlation to the NCA on a quantitative basis. The demonstration of these mediators offers an explanation for the occurrence of local immediate and persistent wheals at sites of bee, wasp, and sea nettle stings in nonsensitized individuals.