Human EJ H-ras or c-sis oncogene-transformed Balb/c 3T3 cells were investigated to discern the oncogene dependence of earliest events of primary tumor development by utilizing bacterial lacZ or human alkaline phosphatase histochemical marker genes. The locations and morphologies of transformed or untransformed Balb/c 3T3 cells, also transfected with either marker gene, were monitored in situ by visual, histochemical staining of skin after subcutaneous injection. Cell numbers remaining in the subcutaneous space were quantitated through newly-developed assays maximizing enzymatic detection of the transfected marker gene-encoded enzymes using luminescent substrates. Either oncogene was effective at facilitating cell establishment within the subcutaneous space while parental 3T3 cells were cleared efficiently. Quantitation by enzymatic assays revealed that the rates of cell loss and establishment differed for the two transformants. Ras-transformed cells exhibited a spindled-shaped morphology within the subcutis. In contrast, sis-transformed cells established a dichotomous morphology, with one subpopulation spreading modestly and another remaining rounded. 3T3 cells which established transiently became well-spread in a manner reminiscent of cells in culture. In conclusion, ras and sis oncogenes aid the parental 3T3 cells to overcome the clearance mechanisms at the subcutaneous injection site and modify 3T3 cell morphology as visualized in situ.