Neutralization of endogenous IL-10 can partially overcome proliferative hyporesponsiveness of G-PBMC in MLC. Unfractionated control PBMC (A) and G-PBMC (B), or purified control CD4 cells (C) and G-PBMC–derived CD4 cells (D) from one donor were used at indicated numbers as responders with 100 × 10³ irradiated (30 Gy) allogeneic PBMC stimulators in MLC. Proliferation was measured on day 5 by ³H-thymidine incorporation in cultures without (solid lines) or with (dashed lines) neutralizing antibodies to IL-10. Values represent the mean ± SEM from triplicate cultures. Shown is one of three experiments with side-by-side comparison of unfractionated and purified CD4 cell responders.