Co-culture at constant cell density of nude mouse spleen cells (by themselves unresponsive to the T-cell mitogen concanavalin A (Con A)), with congenic T-enriched lymphocyte suspensions and Con A caused anomalously high activation of K+ transport (measured by 86Rb uptake) and of incorporation of thymidine into DNA; the expected dilution of these two responses by nude spleen cells did not occur. However, if the nude splenocytes were added immediately prior to assay to the enriched T cells that had been precultured in presence of Con A, the expected dilution of the activated T-cell responses occurred; both 86Rb uptake and thymidine incorporation were reduced proportionally to the degree of dilution of the T cells by the nonresponding cells. These data indicate that during co-culture in presence of Con A there is interaction between the T cells, capable of responding to mitogens, and the nude spleen cells. Attempts to demonstrate a diffusible factor in the supernatants of stimulated T cells were unsuccessful. The measured interaction is sufficient to explain our previous paradoxical findings that enrichment of T cells as measured by membrane markers did not cause a corresponding enrichment for either cation transport or for thymidine incorporation, and that depletion of T cells in the B-enriched cultures did not cause a corresponding decrease in these two Con A induced responses.