The c-myc and N-myc nuclear oncoproteins are implicated in the genesis and maintenance of the transformed phenotype in several types of neoplastic disease, and the c-myc protein is involved in the progression of normal cells through the cell cycle. We have designed and developed sensitive and quantitative ELISAs for these proteins. Myc proteins are captured from cell lysates by an antibody directed against a peptide sequence substantially conserved in all known myc proteins; the captured proteins are recognised by a specific anti-c-myc or anti-N-myc monoclonal antibody conjugated to alkaline phosphatase; bound alkaline phosphatase is measured with an extremely sensitive cycling enzyme system that generates a coloured end-product. The c-myc assay is calibrated using bacterially expressed human c-myc protein. We have used this assay to estimate the number of c-myc molecules in a range of normal and transformed cells of human, murine, and feline origin; to monitor increases in c-myc expression when quiescent cells are stimulated with growth factors; and to follow the decrease in c-myc protein levels when HL60 promyelocytic leukaemia cells are induced to differentiate with dimethylsulphoxide or phorbol esters.