CD4⁺ T cells, whether viable or irradiated, augment proliferation of CD8⁺ T cells. Congenic Thy1.1 CD4⁺ and CD8⁺ T cells derived from TDLN were labeled with CFSE immediately prior to intravenous adoptive transfer into hosts with 10-day pulmonary metastases. Lungs and spleens were harvested on days 1 and 3 and single cell suspensions were analyzed for CFSE staining intensity within the gated Thy1.1 subset. The percentage of cells demonstrating at least 10-fold reduction in CFSE intensity is indicated on the left side and the mean fluorescence intensity is indicated on the right side of each graph.