The effect of urethan on various spleen-lymphocyte functions was studied in the rat for 45 days after administration of the carcinogen. For 3 days following the last injection of urethan, the total cell number and LPS reactivity were greatly reduced. Certain reactivities, probably T-cell dependent functions, such as responsiveness to the in vitro mitogenic effect of PHA, Con A, and primary stimulation by histocompatibility alloantigens in the one-way reaction, were selectively enriched during the first 2 days. Thereafter several dissociations of these lymphocyte functions can be observed: i.e., various intervals succeed during which one function may be enriched and other(s) diminished. It seems that the kinetics of enrichment, decay and recovery of the T-cell subsets involved in the reactions investigated follow a distinctive profile for each one of them. It is suggested, as a working hypothesis, that the unbalanced situations derived from the time-course discordance of the quantitative changes in the various lymphocyte functions may allow, or even enhance, tumor development.