Secretion of plasminogen activators (PA) has been shown to be an important method by which cells can initiate degradation of the extracellular matrix (ECM). In this study we have examined the PA production of two murine cell lines, KHT-LP1, a fibrosarcoma and SCC-VII, a squamous cell carcinoma, and have found a high degree of clonal heterogeneity. Our method for assaying PA activity measures the PA activity of small colonies of cells derived from single cells, using an in vitro fibrin/agarose PA assay in which colonies with PA activity form discernable 'halos' in the fibrin/agarose semisolid growth medium. When these small colonies of cells were disassociated and the component cells were reassayed for PA activity it was again found to be heterogeneous, suggesting that this property can be generated during the growth of the colonies. KHT-LP1 cells derived from single cell clones were assayed for PA activity to determine the rate at which this phenotype was produced. It was found that the rate of formation of the PA activity phenotype was 6.5 x 10(-6) events per cell generation. The component cells of colonies which initially demonstrated high PA activity produced more PA activity than the component cells of the colonies that had low PA activity. This suggests that some aspects of the phenotype may be more stable than others. To examine whether the addition of lethally irradiated cells could stabilize the phenotype we determined whether fibrin/agarose PA assays supplemented with lethally irradiated cells would reduce the heterogeneity of PA activity. The results indicated that the heterogeneity was not reduced, and there was an increase in the average amount of PA activity.