The binding of the chemical carcinogen 3-methylcholanthrene (MCA) was examined in the cytosol fraction from tissues of the rat. With sucrose gradient centrifugation, macromolecular complexes of 3H-MCA were observed in several tissues. A unique binding component was identified in the lung cytosol; other tissues produced a 4S component believed to be at least partially a result of contamination by serum proteins. The lung-binding component had a sedimentation coefficient of about 7S under low ionic conditions. It was altered to a 5S form in the presence of 0.3M KCI, which suggested a subunit composition. The lung component was not found in other tissues tested (serum, liver, heart, kidney, testes, and uterus). These findings were verified by the in vivo administration of 3H-MCA. Although the carcinogen was readily taken up by several tissues, only the lung contained a complex sedimenting in the 7S region of sucrose gradients. Binding specificity has not yet been described, but one other chemical carcinogen dibenz[a,h]anthracene, was also bound by the lung component.