Class II expression following restimulation of a population of T cells which had first been activated and then allowed to revert to small resting cells via IL-2 deprivation was followed on the cell surface and mRNA levels. Our interest was to determine the kinetics of class II expression during in vitro growth, the triggers which can induce them, and also whether similar or different patterns are observed for the three class II antigens, DR, DQ, and DP. The cells responded rapidly to restimulation with conditioned medium containing IL-2 as assessed by increased incorporation of [3H]TdR into DNA. Cell surface expression of HLA-DR reached peak levels by Day 1, and greater than 90% of the cells continued to express DR until Day 4, when the number of positive cells gradually began to decline. The expression of DP also increased, though maximum levels were not reached until Day 3, when it began to decline. The percentage of cells positive for DP was, however, consistently lower than that of DR. DQ was expressed by very few cells, but appeared to increase until Day 4 and then decline. Tac expression was dramatically up-regulated following IL-2 stimulation, remaining high until Day 4, and then declining more precipitously than any of the other antigens. Class I antigen expression was relatively constant during the entire culture period, though a slight decline was noted between Day 5 and 6 when proliferation and viability were at their lowest levels. On the molecular level, DR beta mRNA accumulation peaked at Day 3, then rapidly declined. Reculture in IL-2 on Day 4 resulted in transient reaccumulation of message. Study of DP beta and DQ beta mRNA demonstrated strong expression on Days 3 and 4 and no obvious up-regulation after restimulation with IL-2 on Day 4. The kinetics of Tac mRNA accumulation and the response to restimulation with IL-2 closely resembled that of DR. Finally we compared the ability of different signals to up-regulate class II mRNA and surface expression and to restimulate proliferation. Our results indicated that PHA was more effective then medium alone, but far less effective than was IL-2. PMA was essentially no different than medium alone.