The related proteins p300 and CBP (cAMP-response-element-binding protein (CREB)-binding protein)) are transcriptional co-activators that act with other factors to regulate gene expression(1-5) and play roles in many cell-differentiation add signal transduction pathways(6-10). Both proteins have intrinsic histone-acetyltransferase activity(11,12) and may act directly on chromatin, of which histone is a component, to facilitate transcription. They are also involved in growth control pathways, as shown by their interaction with the tumour-suppressor p53 (refs 13-15) and the viral oncogenes E1A (refs 1, 2, 16) and SV40 T antigen(5). Here we report functional differences of p300 and CBP in vivo. We examined their roles during retinoic-acid-induced differentiation, cell-cycle exit and programmed cell death (apoptosis) of embryonal carcinoma F9 cells(17,20), using hammerhead ribozymes capable of cleaving either p300 or CBP messenger RNAs. F9 cells expressing a p300-specific ribozyme became resistant to retinoic-acid-induced differentiation, whereas cells expressing a CBP-specific ribozyme were unaffected. Similarly, retinoic-acid-induced transcriptional upregulation of the cell-cycle inhibitor p21(Cip1) required normal levels of p300, but not CBP, whereas the reverse was true for p27(Kip1). In contrast, both ribozymes blocked retinoic-acid-induced apoptosis, indicating that both co-activators are required for this process. Thus, despite their similarities, p300 and CBP have distinct functions during retinoic-acid-induced differentiation of F9 cells.