Analysis by means of computerized two-dimensional gel electrophoresis (NEPHGE, IEF) of the [35S]-methionine labeled proteins secreted by normal human MRC-5 fibroblasts revealed 476 polypeptides (258 acidic and 218 basic), many of which appeared as charge trains due to modification. Similar analysis of the proteins secreted by SV40 transformed MRC-5 fibroblasts (MRC-5 V2) showed a striking decrease in the levels of many of these proteins as well as the appearance (or increased synthesis) of 47 polypeptides that were either absent or present in very low amounts in normal cells. Of the major secreted polypeptides whose relative proportion decreased dramatically in the MRC-5 V2 cells, 15 were found to be abundant components of other normal (nontransformed) fibroblasts (W138, Xeroderma pigmentosum cell lines). Low levels of these radioactively labeled polypeptides were observed in transformed human cell lines of fibroblast (W138, SV40, HT1080), epithelial (HeLa, transformed amnion cells (AMA), A431, A459) and myeloid (HL-60) origin. No major secreted polypeptide from MRC-5 V2 cells was synthesized exclusively by the transformed cell lines.