Evaluation of the interrelationship between the type and timing of the second stimulus and expansion of restimulated CD4 (A and B) and CD8 (C and D) T cells. To normalize for the impact of persistent growth attributable to primary stimulation, the ratio of growth after restimulation to growth by matched control cells in the absence of restimulation was calculated. Regardless of the interval between stimulations, by day 3, CD4 (A) and CD8 (B) cells re-incubated with anti-CD3/CD28 beads usually had a stimulation ratio of less than 1 i.e. cells had diminished in number compared to control untreated cells. Even at day 7 (B and D), the stimulation ratio seldom exceeded 1 for cells which had been rested in vitro between stimulations for less than 20 days. By comparison, the stimulation ratio for anti-CD3/MMC treated cells seldom dropped below 1 at day 3, and increased further with time, even when the rest interval between stimulations was less than 14 days.