The effect of Reverin (pyrrolidino-methyl-tetracyeline) an inhibitor of protein synthesis on radiation damage (mainly in the head region) in 9-day-old rat embryos has been investigated and compared with its effects on other cell systems. An administration of 91.0 mg/kg body weight Reverin alone had a teratogenic effect. If Reverin was injected i.m. or i.p. 1 hour before irradiation the number of eye anomalies in 13-day-old fetuses was increased from 25.8 % (irradiation only) to 58.8 %.This enhanced effect was not due to an additive but to a synergistic mechanism. However Reverin failed to show synergistic effects on growth of Ehrlich carcinoma cells in ascites-and solid form as on chromosomal aberrations of normal chinese hamster cells, the death rate of irradiated chinese hamster cells was sensitized. It is proposed that tetracyclines act mainly on the immediate repair by blocking repair enzyme systems or by depression of energy metabolism. Differences in the reactions of other cell systems can be explained by a different biological relevance or vulnerability of repair systems.