Dose-response relationship between secreted PR3 and reduction of S-phase of CFU-GM. RBL/PR3 transfectant cells were grown for 3, 6, 24, and 72 hours and the resulting CM tested for S-phase–reducing activity. The concentration of PR3 in ng/mL was measured by ELISA. The number of colonies per dish in the control was 268 ± 32 (mean ± SD) without and 175 ± 21 with cytosine arabinoside, respectively.