The expression of Thy 1.2 (theta C3H) antigen was measured on the membranes of normal and neoplastic RIII and C3H mammary cells. Competitive inhibition assays revealed that the average membrane content of Thy 1.2 in mammary tissues was about equal to that of lymph node cells. Higher percentages of Thy 1.2-positive cells than mammary tumor virus (MuMTV)-positive cells were observed by immunofluorescence, which suggested that not all the Thy 1.2-positive cells recovered from tumors were also MuMTV-positive. Established tissue culture cell lines C3H and RIII MT expressed lower levels of Thy 1.2 than did cells from mammary tumors. Treatment with the synthetic gluco-corticoid dexamethasone increased the average Thy 1.2 expression in cultured mammary tumor cells as well as the levels of RNA-directed DNA polymerase. Since the percentages of Thy 1.2-positive cells also were greater in steroid-treated cultures, while fewer cells were needed to absorb a standard amount of anti-Thy 1.2 activity, it was concluded that dexamethasone enhanced membrane Thy 1.2 expression as well as MuMTV production by the cultured cells.