Two hemorrhagic proteinases from Crotalus atrox venom, hemorrhagic toxin c (Ht-c) and hemorrhagic toxin d (Ht-d), were characterized and compared to one another. The two toxins are zinc metalloproteinases which both have molecular weights of 24,000. Their isoelectric points are slightly acidic, Ht-c being the more basic of the two with an isoelectric point of 6.2, whereas Ht-d has an isoelectric point of 6.1. Only minor differences were found in the amino acid compositions of the two toxins. The toxins were both demonstrated to be hemorrhagic, using an in vivo assay, and also proteolytic. Prior treatment of the hemorrhagic proteinases with ethylenediaminetetraacetic acid and o-phenanthroline eliminated both the hemorrhagic and the proteolytic activities. Aprotinin and phenylmethylsulfonyl fluoride had no effect upon these activities. The pH optimum of the proteolysis by Ht-c and Ht-d on hide powder azure as the substrate was between pH 8 and pH 9. The circular dichroism spectra for Ht-c and Ht-d appear almost identical with respect to minima positions and elipticities, indicative of very similar solution structures for the two enzymes. Antiserum raised in mice against Ht-c was assayed on double-diffusion Ouchterlony plates for cross-reactivity with other hemorrhagic toxins from C. atrox venom. From this experiment it was concluded that the two hemorrhagic proteinases Ht-c and Ht-d share identical antigenic structures. This was corroborated by tryptic mapping of the two toxins. Only one major difference was observed from the maps. In the case of Ht-c, it was determined that an aspartate was substituted by an alanine when compared to Ht-d. From these characterization studies we conclude that Ht-c and Ht-d are isoenzymes with only very minor differences in their structures.