Activation of lymphocytes was assessed by measuring the enhanced labeling with 5-3H-uridine of cell suspensions from various rat lymphoid organs produced by phytohemagglutinin (PHA) in a 4-hr period. No evidence was obtained for reduced function of thymus cells when compared with cells from other lymphoid organs over this time period. Suspensions from lymph nodes were more strongly activated by PHA than were suspensions from spleen. Suspensions from the thymus either failed to be activated or were only weakly activated. The results indicate that the widely reported failure of the majority of thymus lymphocytes to be transformed by PHA reflects a failure of cell activation rather than of cell survival in vitro.