Overexpression of the nuclear phosphoprotein p53 has been detected in many different transformed human cell lines and primary adult tumors. Elevated steady-state levels of p53 appear to be the result of an increase in the stability of the protein and, in adult cancers, high levels of the protein are associated with mutation of the p53 gene. In this study, overexpression of p53 was detected in 4 out of 5 human neuroblastoma-derived cell lines. The protein expressed by each of these four lines had a significantly prolonged half-life relative to the p53 protein in immortalized rodent fibroblasts and normal bovine adrenal medullary cells. However, no mutations were detected in the highly conserved regions of the p53 gene in these four neuroblastoma lines and the protein being expressed was not recognized by the mutant-specific anti-p53 monoclonal antibody, PAb 240. Upon retinoic acid-induced differentiation of the LA-N-5 neuroblastoma cell line, the level of p53 protein declined, as did the level of p53 mRNA, but the half-life of the protein remained unchanged. The high level of protein observed in the undifferentiated cell lines appears to result from expression of a stable wild-type p53 protein and increased transcription. In contrast, p53 protein was undetectable in two neuroepithelioma-derived cell lines; the p53 gene in one of these lines contained a nonsense mutation, while the other transcribed truncated p53 mRNA.