Reversibility and modulation by CSF of PR3 activity. (A) Shows CFU-GM in S-phase after a 2-hour incubation with PR3 CM followed by washing of the cells and continued incubation in fresh medium for another 20 hours before addition of cytosine arabinoside. The middle bar shows CFU-GM in S-phase at 2 hours when incubation with PR3 was interrupted. Mean ± SD of three experiments. (B) Shows 2 to 5 hours’ incubation with PR3 CM with and without G-CSF present at 20 ng/mL. Mean ± SD of five experiments. (C) Shows similar experiments with 20 hours’ incubation with and without G-CSF or GM-CSF present at 20 ng/mL before addition of cytosine arabinoside. Mean ± SD of five (G-CSF) and three (GM-CSF) experiments. The asterisk denotes statistically significant reduction of S-phase in the positive controls (P< .01). The concentration of PR3 in these experiments was 1.5 to 7.5 ng/mL. The number of colonies per dish in the controls in these experiments ranged from 140 to 343, median, 244. G-CSF or GM-CSF in itself did not change the fraction of CFU-GM in S-phase (data not shown).