The anti-tumor effect of CTLs on autologous tumor xenografts is dependent on the time of administration, but not the number of doses. A. Mice (5/group) received 5 × 105 tumor cells (i.v.). Once tumors were established in the lung, three of these groups (5 mice/group) were injected with CTLs (5 × 106): Group III on day 5, Group IV on day 15 and Group V on both days 5 and 15. (The control group I did not receive tumor, Group II received tumor but not CTLs). At day 30, mice were sacrificed and lungs were excised and weighed. Differences in lung weight between treatment groups were evaluated by the Student t-test: I vs. II, P < 0.001; I vs. III, P < 0.1; I vs. IV, P < 0.001; II vs. IV, P < 0.35; III vs. IV, P < 0.001. B. Mice (groups II-IV, 5 mice/group) were injected with 5 × 105 tumor cells (i.v.). After lung tumors were established, 4 groups (III-VI) received CTLs (5 × 106) as follows: Group III – one dose of CTL on day 3; Group IV – days 3 and 4; Group V-days 3, 4 and 5; Group VI-days 3, 4, 5 and 6. Normal lung weight was derived from Group I, which did not receive tumor cells; Group II received PBS and no CTL. On day 30, mice were sacrificed and lungs were excised and weighed. The growth of lung tumors in mice treated with tumor specific CTLs was significantly inhibited by one dose on day 3; there was no advantage to the administration of additional CTL doses.