Fetal bovine and dog serum were selectively freed of plasminogen by affinity chromatography. The resulting serum as well as native and reconstituted serum (obtained by the addition of purified plasminogen to the plasminogen-depleted serum) were used to examine the role of plasminogen in (a) growth of normal and SV-40-transformed hamster embryo fibroblasts in liquid medium, (b) growth of SV-40-transformed hamster embryo fibroblasts in soft agar, (c) aggregation - a characteristic morphological change of SV-40-transformed hamster cells, and (d) migration of SV-40-transformed and control 3T3 cells from a monolayer into a "wound." The results demonstrated that exponential growth of both normal and transformed cells in liquid medium proceeded at the same rate in the presence or absence of plasminogen. In contrast, removal of plasminogen markedly depressed the plating efficiency of transformed cells in soft agar, eliminated their characteristic aggregation, and substantially reduced the extent of migration. The role of plasminogen and its activation in oncogenic transformation is discussed.