The incorporation of [3H]thymidine by rabbit chondrocytes in vitro has been developed as a sensitive assay for plasma somatomedin. A concentration of normal plasma of 2-5% enhanced [3H]thymidine incorporation by 5- to 20-fold compared with basal levels in the absence of plasma. The mean potency of plasma from normal adult men was 0-96+/-0-1 u./ml (mean+/-S.D.) and from acromegalic patients 1-9+/-0-4 u./ml. The apparent potency of hypopituitary plasma alone increased on heating which suggested the presence of heat-labile inhibitors of somatomedin activity. The potency of heated hypopituitary plasma (0-6+/-0-09 u./ml) remained significantly lower (P less than 0-01) than normal plasma. Human growth hormone (0-1-20 muu./ml), bovine growth hormone (0-5 20 muu./ml), insulin (0-5-5 muu./ml) and glucose (0-3-2 mmol/1) had no direct effect on the incorporation of [3H]thymidine. Chondrocytes which had been previously stored frozen also showed a response to plasma somatomedin.