We have shown in previous studies that the cytotoxic effector cells in in-vitro-induced cytotoxicity against herpes-simplex-virus-infected targets are principally T gamma cells and that they derive from theophylline-resistant T mu cells. In the present work, we studied the effect of theophylline on the induction and expression of cytotoxic activity in T-cell subsets. When T cells were prefractionated before the induction phase, the theophylline-resistant (Thr) subset, as determined by E-rosetting, was unaffected by the presence of theophylline during sensitization, whereas T mu and unfractionated T cells were inhibited. On the other hand, when theophylline was present during the cytotoxicity assay, all T-cell subsets were similarly affected. Studies of kinetics revealed that a short preincubation with the drug had an enhancing rather than an inhibitory effect on both the proportions of T gamma and Thr and on their cytotoxic activity at 3 days.