Bovine parathyroid hormone labeled with tritium on the methionine residues by [3H]methyl exchange ([3H]bPTH) was administered intravenously to 35-day-old mice and localized in tissues by light microscope autoradiography. After 10 min most of the [3H]bPTH had been taken up from plasma. In the kidney and liver, radioactivity was not displaced by simultaneous administration of unlabeled bPTH, and was as intense after giving oxidized [3H]bPTH ([3H]ox bPTH) as [3H]bPTH, suggesting that binding was largely associated with nonspecific processes. In long bones, [3H]bPTH was bound to osteoblasts and preosteoblasts lining the endosteal and periosteal surfaces of compact bone. In the area of the growth-plate there was intense labeling on new endochondrial bone, and on hypertrophic chondrocytes in the region of calcification. There was little labeling in marrow and, in contrast to other tissues, much reduced binding was seen when a large excess of unlabeled parathyroid hormone was administered together with [3H]bPTH, or when [3H]ox bPTH was given. It is concluded that binding of parathyroid hormone to cells in bone is largely specific, and that the hormone has a function in cartilage which relates to the differentiation and calcification of chondrocytes of the growth-plate that occurs prior to its replacement by new bone tissue.