The BALB/3T3 clone A31 mouse embryo cell line has been used by many investigators as a model "normal" "fibroblast" line for a variety in vitro studies. It has been shown, however, that these cells are not "normal" because they will produce tumors within 2-4 months if 3 x 10(4) cells are implanted subcutaneously in BALB/c mice attached to 0.2 x 5 x 10-mm plastic plates. Previous studies also suggested that these cells were not fibroblasts because they gave rise tumors with the characteristics of vascular endothelium not fibroblasts. We now report that BALB/3T3 (clone A31), BALB/3T3-T, a proadipocyte subclone of clone A31 cells, and six recent subclones of BALB/3T3-T cells show additional differentiation patterns were tumors derived by implantation of these cells attached to plastic plates are examined. Differentiation into pericytes, chondrocytes, and fibroblasts was observed. We conclude that the BALB/3T3 clone A31 cell line and related lines are multipotent mesenchymal cells which are capable of differentiation into a variety of cell types.