Cell cultures of human skin fibroblasts, myoblasts, and fused muscle cells were grown in the presence of [14C]leucine or a mixture of [14C]amino acids. The proteins synthesised and secreted or leaked into the culture medium during radio-labelling were separated by one- and two-dimensional PAGE and detected by fluorography. Four major bands of Mr 54 kD, 52 kD, 51 kD, and 49 kD were present at greatly increased concentration in fibroblast media. These fibroblast-specific polypeptides can be readily detected in myoblast/fibroblast cocultures with fibroblast content as low as 5%.