Clonal interaction among two subpopulations of Ehrlich ascites carcinoma was studied during in vivo growth in immune competent N/D mice in which the cell lines had been propagated for several years as ascites tumors. A growth inhibitory interaction by a subcutaneously slow growing subpopulation (E1.15) on a fast growing subpopulation (E1.95) was demonstrated only when the cells had contact during solid tumor growth. The effect was dependent on the relative proportion of the suppressing cell line. An identical effect was exerted by radiation killed inhibitor cells. The inhibition was only transient. If the tumor cell lines were grown intraperitoneally as ascites tumors without cellular contact, no interaction was found using flow cytometric DNA analysis to determine alterations in the relative proportions of the cell lines. Ascites from the inhibitor cell line E1.15 had no inhibitory effect on E1.95. Pre-immunization with radiation killed E1.15 cells or simultaneous growth of E1.15 in the opposite flank did not affect the growth of E1.95 significantly. A mononuclear cell infiltrate was found to surround the subcutaneously growing E1.15 tumors in immune competent N/D mice. This was not the case in T-lymphocyte deficient athymic nude mice in which E1.15 grew without delay subcutaneously. It is suggested that the E1.15 cell line was able to elicit a T-lymphocyte immune response only when grown subcutaneously, and that the close contact between E1.15 and E1.95 in mixed tumors would induce a non-specific growth inhibition of E1.95 cells which themselves were not able to induce a T-lymphocyte response.