A set of monoclonal antibodies was constructed by immunising mice with peptide fragments of the c-myc oncogene product. One such antibody, Myc 1-6E10 was shown to bind to a 62,000 dalton protein identifiable with the c-myc product (p62c-myc). The antigen recognised was not destroyed by paraffin wax embedding. Myc 1-6E10 was used to characterise the distribution of p62c-myc in archival testicular tumour material. Normal testes expressed only small amounts of p62c-myc. Seminomas showed increased nuclear and cytoplasmic staining. Undifferentiated teratoma showed little activity, whereas p62c-myc was abundant in the nuclei of differentiating epithelial structures, yolk sacs and embryoid bodies. Only small amounts of p62c-myc were seen in the tumours of 5 patients who subsequently died from their disease.