The spicule venoms of Euproctis chrysorrhoea and Euproctis subflava were investigated for their capacity to hydrolyze chromogenic tripeptide substrates with selective affinities for various serine proteases. Seven substrates were assayed with affinities for trypsin and thrombin, trypsin and urokinase, serine proteases, chymotrypsin, glandular kallikrein, plasma kallikrein and plasmin. Venom material has a broad spectrum of affinities for the substrates with relative high plasma kallikrein activities. In E. chrysorrhoea venom, trypsin-like activities predominated, whereas E. subflava venom hydrolyzed, in preference, substrates with an affinity for chymotrypsin. The venoms were fractionated on Sephadex G-100, leading to three fractions, all having serine protease activity. The ratios of substrate specificities were markedly different, indicating that in both caterpillar venom preparations at least two separate serine proteases are present. In addition, in human plasma, inhibitor activity could be detected to the kallikrein activity of E. chrysorrhoea, but not of E. subflava. The trypsin-like activity was not inhibited by human plasma. These and earlier studies warrant the assumption that serine proteases, particularly kallikrein, are major factors in the elicitation of clinical symptoms observed after contact with caterpillar spicules.