Cytotoxic T lymphocyte (CTL) morphology and function was examined in beige (bg/bg) mutant mice during infection with lymphocytic choriomeningitis virus (LCMV). Virus-specific, class I-restricted CTL activity mediated by total spleen leukocytes isolated from bg/+ or +/+ mice on days 7 or 9 postinfection with LCMV was moderately higher than that mediated by spleen cells isolated from bg/bg mice. The CTL generated in bg/bg mice had aberrant morphology. Lyt-2+ cells isolated from bg/+ or +/+ mice had typical large granular lymphocyte (LGL) morphology and contained numerous small azurophilic granules, whereas Lyt-2+ cells isolated from bg/bg mice contained only one or two large atypical granules in their cytoplasm. Aberrant LGL morphology correlated with reduced lytic capacity. The bg/bg CTL were inefficient killer cells mediating, on a per cell basis, only one fourth of the lysis mediated by bg/+ CTL. The bg/bg mice appeared to mount a compensatory response to regulate virus replication, because frequencies of Lyt-2+ cells and cells that specifically bound to virus-infected target cells were elevated as compared with their frequencies in bg/+ mice. The higher proportion of the CTL phenotype cells appeared to be a consequence of expanded proliferation of Lyt-2+ cells. These results demonstrate that, in comparison with bg/+ and +/+ mice, bg/bg mice have CTL with reduced lytic capacities, but may compensate during virus infection by expanding the number of these cells. Furthermore, these data suggest that the depressed lytic activity may be a consequence of aberrant granule formation.