Cellular interactions between three subpopulations of Ehrlich ascites tumor and between these and the P388 murine leukemia were studied during growth of solid tumors obtained by mixtures of the cells in immune competent N/D mice. An immunogenic Ehrlich cell line (E1.15) induced an immunologically based growth inhibition of the two other Ehrlich cell lines (E1.80 and E1.95) which themselves were non-immunogenic. E1.15 was, however, unable to induce an immunological response against the P388 cell line. It is therefore suggested that when in close contact, immunologically induced cellular responses imposed by an immunogenic cell line on other cell lines require genetic and thereby close immunogenic resemblance between the cell lines. Another type of interaction was found between the E1.95 cell line and the P388 line which showed nearly identical growth characteristics as determined by tumor weight day 14, tumor growth curves, cell cycle times (per cent labelled mitoses) and cell cycle distributions (flow cytometric DNA analysis). After 2 weeks of growth of mixed P388/E1.95 tumors, flow cytometric DNA analysis on fine-needle tumor aspirates showed nearly total dominance of P388. This type of interaction required close cellular contact of viable cells, and no cellular immune response was elicited by the host animals. A third finding was that a faster growing Ehrlich cell line E1.95 dominated the tumors when inoculated in mixture with a slower growing subpopulation E1.80. This could be explained on the basis of the cell kinetic differences between these two cell lines.