AbstractDifferentiation of circulating erythrocytes was inhibited by all‐trans‐retinoic acid in 8‐ and 9‐day‐old mouse embryos exposed in utero on day 8 of gestation. Histochemical, light, and electron microscopic examinations revealed that all‐trans‐retinoic acid first decreased the numbers of polychromatic erythroblasts and then increased them. Simultaneously, immature erythroblasts proliferated, and differentiation of mature primitive erythrocytes was suppressed. Electron microscopy of these immature erythroblasts revealed monosomes rather than the usual polyribosomes in the cytoplasm. RNA histochemistry revealed a greater number of intermediate differentiating erythroblasts with pyronin‐positive cytoplasm than those in the controls and revealed also a dose‐dependent relationship between the number of polychromatic erythroblasts with positive pyronin staining and the controls. These findings suggest that all‐trans‐retinoic acid destroys the messenger RNA system, resulting in an inability