In order to find out reliable criteria to conclude on the prognosis of transplanted cartilage the quality of isolated rabbit chondrocytes was assayed in vitro. Using a semisolid medium a culture system is introduced, which is able to detect colony forming units of chondrocytes. Furthermore their proliferative and differentiating capacity was investigated. Colony formation in vitro follows a strict dose response relationship. An inverse relationship was observed between the age of the donor and the colony incidence in vitro. Cells from neonatal cartilage could be induced to grow as fibroblastic colonies in the presence of conditioned medium from fibroblastic monolayer cultures. No effect, however, could be observed with chondrocyte derived conditioned medium.