Screening for retinoic acid-binding protein (RABP) in experimental tumors revealed the presence of this protein in 3 mammary tumors, 2 metastatic colon tumors, B16 melanoma, Lewis lung carcinoma, Ridgway osteogenic sarcoma and keratoacanthoma. RABP was below the limits of detection in 2 weakly metastatic colon tumors and sarcoma 180. After s.c. implantation of RABP-containing tumors into mice, this protein could be traced in the lungs due to pulmonary metastases. Following implantation of Lewis lung tumors, RABP was detected in the lung on the 6th day. On the 15th day after implantation, RABP was present in lung and brain, but not in other tissues where this protein was normally lacking. In primary cultures of Lewis lung carcinoma, the lower limit for detection of RABP by sucrose gradient sedimentation corresponded to 0.12 mg protein that was extractable from 3 .times. 105 cells. Chick embryo and rabbit ear skin extracts contained RABP. The level of cellular retinol-binding protein was high in chick embryo skin but only marginal in rabbit ear. The amounts of these proteins on chick embryo and rabbit ear skin correlate with the biological potency of retinol and retinoic acid, as observed by others.