AbstractThe eggs ofCalotes versicolorwere exposed to different concentrations of trypan blue around the time of appearance of limb buds. The treatment resulted in malformations of limbs, hemorrhages in various locations, microphthalmia, kinky tail, and retarded development and growth of the contained embryos. The proportion of affected embryos varied significantly from clutch to clutch, perhaps due to genetic variation. All treated embryos, including those apparently unaffected, had trypan blue in the blood circulation, but not in the amniotic cavity or extraembryonic celom. Trypan blue was seen in kidney, liver, and other organs in decreasing order of frequency and concentration. On histological examination a change in the mitotic and necrotic frequency of the cells was noticed as one of the early events. The initial necrosis was close to blood vessels. The protective role of yolk sac in trypan blue‐induced teratogenesis is discussed and contrary to the widely held view it is concluded that the yolk sac appears to play no such significant role inCalotes.It is suggested that trypan blue directly affectsCalotesembryos, perhaps by altering cell permeabilit