Expression of the cyclin D1 gene is induced when quiescent fibroblasts are stimulated to reenter the cell cycle by addition of growth factors. Moderate ectopic expression of cyclin D1 in early G1 facilitates progression through G1. When transiently overexpressed at the G1/S boundary, cyclin D1 prevents S phase entry, suggesting a dual role for this protein in cellular growth control. It was shown that the retinoblastoma protein (pRB) can activate cyclin D1 gene expression; furthermore, there is evidence that expression of the cyclin D1 gene is down-regulated by the SV40 large T and adenovirus E1A genes, both of which were shown to target pRB. We now report that in diploid human fibroblasts functional inactivation of pRB by adenovirus E1A is not sufficient for efficient repression of cyclin D1 gene expression, since the E1B gene product, in addition to E1A, is required for repression of the cyclin D1 gene. Since E1B was shown to target p53, we investigated the role of p53 for expression of the cyclin D1 gene. In a cell line with temperature-sensitive p53, cyclin D1 is moderately expressed at the restrictive temperature. Induction of p53 function by temperature shift leads to an increase of cyclin D1 mRNA and protein, parallel to the activation of p21WAF-1/CIP1 gene expression in this system. When the capability of adenovirus gene products to affect expression of either gene was analysed, we found that infection of Ad5 drastically reduced cyclin D1 and p21WAF-1/CIP1 gene expression in cells where p53 function is limiting. Under these conditions E1A and E1B cooperate to reduce the cyclin D1 level, while p21WAF-1/CIP1 expression was found insensitive to E1A expression. In cells containing elevated p53 function, modulation of gene expression by E1B was severely compromised; under these conditions, expression of E1A reduced expression of cyclin D1 without affecting p21WAF-1/CIP1. The data suggest that E1A and E1B cooperate to inhibit expression of cyclin D1 and identify the cyclin D1 gene as a new downstream target for p53.