Two intracellular proteins that bind compounds with vitamin A activity have been discovered in animal tissues. One, called cellular retinol-binding protein (CRBP), binds retinol with high specificity and affinity, but not retinal or retinoic acid. The other protein, called cellular retinoic acid-binding protein (CRABP), has high affinity for retinoic acid but does not bind retinol or retinal. CRBP is different from the well-known serum retinol binding protein. The proteins are present in many fetal tissues, whereas their tissue distribution in the adult rat differs. The levels of these proteins change differently during perinatal development, suggesting that they are regulated in a nonsynchronous manner. Some malignant tumors contain these proteins. The presence of these proteins could be an indication of whether the tumor might be inhibited by or might require vitamin A for growth. It appears that the cell nucleus is a target for retinol action, as CRBP allows specific interaction of retinol with the nucleus, showing the presence of specific binding sites for retinol. The number of these sites is dependent on the vitamin A status of the animal.