Many human cancers are characterized by mutations of p53, a nuclear phosphoprotein which controls elements of the cell cycle. Turnover of p53 in normal cells is rapid, and the minute quantities of protein that are usually present are not detected by immunocytochemical methods. Mutations of the p53 gene in tumour cells are associated with a slower turnover and subsequent accumulation of the protein in both nucleus and cytoplasm. Genetic abnormalities of the short arm of chromosome 17, which is the site of the p53 gene locus, are a feature of astrocytic tumours. Using a panel of five antibodies to p53 and a standard immunocytochemical method, we found detectable quantities of p53 in the cells of 3/16 diffuse astrocytomas, 8/14 anaplastic astrocytomas, and 24/34 glioblastoma multiforme. Progression of one patient's tumour from a diffuse to an anaplastic astrocytoma was characterized by the accumulation of p53. The more malignant histological features of anaplastic astrocytoma and glioblastoma multiforme appear to be reflected by a greater incidence of p53 accumulation.