Chondrocytes freshly explanted from the sternal cartilage of 14- to 16-day-old chick embryos proliferate and differentiate in cell culture in a serum-free medium as well as in a medium containing 10% foetal calf serum. A comparable degree of multiplication and differentiation was found in chondrocytes cultivated in the serum-free medium containing native growth-promoting alpha-globulin (GPAG). The degree of proliferation was smaller in chondrocytes cultivated in a modified Eagle MEM which is fully chemically defined and contains low molecular weight substrates only. Since this medium does not contain either hormones or proteins, it is suitable for the cultivation of chondrocytes which should be employed when studying the mechanism of the effect of hormones which influence chondrocyte proliferation or chondrogenesis.