Retinoic acid decreases the kinase activity associated with cyclin E and cyclin A. Mitogenic stimulation and retinoic acid treatment of the B cells were performed as described in the legend to Fig 3B. Whole cell extracts were prepared, immunoprecipitated with antibodies against cyclin E (top) or cyclin A (bottom), and then the associated kinase activity of the immunoprecipitates was determined with histone H1 as substrate. IP, immunoprecipitating antibody; μ, F(ab′)₂ fragment of rabbit antihuman IgM; SAC,Staphylococcus aureus crude cell suspension; RA, retinoic acid.