Worldwide usage of different colouring agents in the food industry prompted us to study their toxicity. The potential adverse effects of erythrosine (ER, FD & C Red No. 3) on the spermatogenesis process were investigated in adult mice. Testicular lactic dehydrogenase isoenzyme activity (LDH-X), a pachytene spermatocyte marker of testicular toxicity, was significantly decreased to 71.8% and 68.6% of the control value after daily p.o. administration of ER (21 days) in doses of 68 and 136 mg kg-1 respectively. At the same time, the normal average epididymal sperm count as well as the percentage of motile sperms were significantly inhibited by about 50% and 57% respectively. Moreover, ER was shown to disrupt the normal morphology of the sperm head. Thus, after 5 daily p.o administrations of ER in doses of 680 and 1360 mg kg-1 (equivalent to 10 and 20% of its LD50) it increased the incidence of sperms with abnormal head by about 57% and 65% respectively. The induced increase in sperm abnormalities could enhance the spermatogenic dysfunction and germ cell mutagenicity. These findings indicate that ER in the used doses has a potential toxic effect on spermatogenesis in mice and in turn, it may affect its testicular function and reproductive performance.