This study investigates the human oncoprotein MDM2, which interferes with regulation of cell division and apoptosis. Fifteen mixed-type follicular non-Hodgkin's lymphomas, ten leukaemias, two hepatocellular carcinomas, one osteosarcoma, and ten normal cell lines (fibroblasts, osteoblasts, mesothelium, peripheral lymphocytes) were tested for MDM2 expression and MDM2 gene mutation by reverse transcriptase-polymerase chain reaction (RT-PCR), immunocytochemistry, and nucleotide sequence analysis. Two follicular lymphomas, three leukaemias, both hepatocellular carcinomas, and the osteosarcoma sample showed transcription of the activated MDM2 gene. These samples lacked amplified MDM2 genes and carried mis-sense, non-sense and frame-shift mutations in a zinc finger region of MDM2, altering the amino acid sequence or causing premature termination of transcription. The mis-sense mutations were found in tumour cells that showed significant accumulation of MDM2 and lack of nuclear p53. Non-sense mutations and frame-shift mutations were found in tumours lacking MDM2 proteins. The mutations may affect the biological properties of MDM2 proteins.