Murine splenic T cells undergo apoptosis when the TCR complex is re-cross-linked in the absence of costimulation during a primary immune response. However, if the CD28 complex is also cross-linked, growth continues without induction of apoptosis. Prevention of apoptosis by CD28 costimulation was associated with increased expression of bcl-xL, while overexpression of bcl-2 in T cells from bcl-2 transgenic mice was not protective. In both situations, surviving cells can be recovered in a growth arrested state following the primary response, many more if CD28 was also religated. When these cells were restimulated in secondary response, those surviving TCR religation without CD28 costimulation could not be induced to proliferate further. In contrast, cells given CD28 costimulation during the primary response proliferated well after restimulation. Thus, the CD28 signaling pathway may function not only in the initial activation of naive T cells, but also in maintaining their viability and responsiveness during a primary immune response. In addition, the results further suggest that bcl-2 and bcl-xL regulate T cell survival under different conditions, with bcl-xL being perhaps more important in maintaining viability of activated T cells traversing the cell cycle.