The RAR-selective antagonist Ro 41-5253 blocks the effects of retinoic acid. (A) The effect of Ro 41-5253 on DNA synthesis was measured as described in Materials and Methods, using Ro 41-5253 at a concentration of 1 μmol/L, and retinoic acid at the concentration of 10 nmol/L. (B) B cells (1.5 × 10⁶/mL) were stimulated with μ (37 μg/mL) and SAC (0.005%) for 32 hours in the absence or presence of retinoic acid (10 nmol/L). For Ro 41-5253 treatment of the cells, Ro 41-5253 (0.5 or 1 μmol/L) was added to the cell cultures alone or together with retinoic acid and the cells were harvested at 32 hours. Total cell lysates were prepared and subjected to Western blot analysis for pRB as described in Material and Methods. (C) Ro 41-5253 (1 μmol/L) was added to the cell cultures alone or together with retinoic acid (10 nmol/L) at the indicated times before harvesting the cells at 32 hours. Total cell lysates were prepared and subjected to Western blot analysis for p21^Cip1. μ, F(ab′)₂ fragment of rabbit antihuman IgM; SAC,Staphylococcus aureus crude cell suspension.