Many host factors contribute to the death of cells shed from cancers and with many immunogenic tumors immune control is decisive in determining the incidence of metastasis. Although an aromatic analogue of retinoic acid did not affect the growth of nonimmunogenic carcinomas or sarcomas it slowed the tumor growth of immunogenic cancers but only if they were grown in immunocompetent hosts. A working hypothesis is that retinoids enhance the magnitude of a T-cell dependant host response to the tumor and it is this that causes the inhibition of growth. If the animals are rendered incapable of evoking a T-cell response then retinoids are unable to influence tumor growth.