Primary cultures of fibroblast-like cells were obtained from skin and articular cartilage of human donors in the age bracket of 1 to 15 years. For growth these cultures required 1 mg/liter of cholesterol added to Medium A2 plus acetyl choline, Na pyruvate, and D-galactosamine HCl (APG) containing 10% lipoprotein-free human serum. Established cell lines did not require cholesterol for growth. Eagle's medium could be used in place of Medium A2 plus APG with the same results. Desmosterol could replace cholesterol but lansterol or 7 dehydrocholesterol could not. Other cholesterol precursors were tested and found to be inactive. With the proviso that cholesterol precursors entered the cell and had to be converted to cholesterol to function, it was concluded that the particular primaries studied lacked a functional enzyme system to reduce the double bond at carbon 7.