Proliferation of preG- and G-CD4 cells in response to increasing concentrations of immobilized CD3-specific IgM in the presence of IL-2. PreG- (solid lines) or G-CD4 (dashed lines) cells (2.0 × 10⁴/well) were seeded into flat-bottom 96-well plates precoated with antibody 38.1 at the concentrations shown. Values for ³H-thymidine incorporation represent the mean cpm ± SEM for triplicate cultures.