A developed mixture testing protocol, evaluating malformation of Xenopus embryos, was used to examine the joint actions of semicarbazide (SC) and thiosemicarbazide (TSC) with benzoic hydrazide (BZH). These compounds were selected to evaluate the effect of a substitution of sulfur for oxygen on osteolathyrism induced in Xenopus embryos. Binary combinations of SC with BZH and TSC with BZH were tested using three mixture ratios for each combination. Each compound was simultaneously tested alone in 96 h static-renewal tests, starting with late-blastula stage embryos. Toxic unit analysis, evaluating induction of malformations in general and osteolathyrism specifically, indicated the joint action for each combination was concentration addition. The results indicate that the three chemicals are similar and non-interactive and are likely to induce osteolathyrism by the same primary mechanism of action. ©1993 Wiley-Liss, Inc.