We describe a procedure for molecular mass determination of hyaluronidases present in animal venoms from different families. Hyaluronidases can be revealed, following their electrophoretic separation in sodium dodecyl sulfate-polyacrylamide gel containing hyaluronic acid, by incubating the gel in Triton X-100 to remove sodium dodecyl sulfate and restore in situ enzyme activity. This method allows the detection of as little as 0.025 turbidity-reducing units after 2 hr incubation. All the hyaluronidases from the analyzed invertebrate venoms had a mass below 50,000 and showed only one component, while those from vertebrate venoms were more than 60,000 and in many instances contained more than one form.