The unique capacity of monoclonal antibodies to distinguish between cells that morphologically appear identical is being employed to gain new insights into the differentiation pathways of human connective tissue cells. This information has implications for the more accurate classification of human sarcomas and human connective tissue diseases. More than 10 monoclonal antibodies have been prepared that are specific for human connective tissue cells. Each of these has been screened with panels of tissue culture cell lines. Of the first four of these further tested by immunohistologic methods, all proved to stain only connective tissue in frozen tissue sections. Each antibody has a unique specificity as demonstrated by its reaction pattern. Some stain most connective tissues, while others appear to be specific for connective tissues of certain organs, and a third category stains the cells that are in their early stages of differentiation. Biochemical characterization of each of these ten antibodies has been initiated. The antigens they detect fall into at least two groups: those in the Mr = 70 to 80 kilodalton region and others above 20 kilodaltons. The larger molecules may represent subspecies of fibronectin. Work continues on the isolation of additional monoclonal antibodies and the further characterization of those already in hand. Multiple antibodies are now being studied on panels of normal, human adult, and fetal tissues as well as of carcinomas and sarcomas. This project appears to be moving forward effectively and meeting its goals to explore the diversity of connective tissue cells. (2)