Supernatant derived from PR1-pulsed CTL1 has no effect on CFU-GM. Day 20 PR1-pulsed CTL1 inhibit CFU-GM from P3 (CML-BC) by 98% compared to P3 cultured without CTL1 (P = .003). CTL1 did not inhibit CFU-GM from D3, the HLA identical normal marrow donor to patient P3. Supernatant taken after a 4-hour incubation of CTL1 with P3 and added back to fresh P3 did not inhibit CFU-GM, indicating that cell contact between CTL1 and targets is required for colony inhibition. Effectors were incubated with target cells at E:T ratio of 5:1 for 4 hours at 37°C before plating in methylcellulose, and colonies were counted on day 14 of culture. Three replicate wells were used to determine CFU-GM and data are displayed as mean colony counts ± standard deviation.