A sensitive radioimmunoassay for human c-myc protein was developed by using an amino-terminal fragment of c-myc protein, c-myc(11-24). Immunoreactive c-myc protein was found to be present in 3 human tumors (HL-60, N231, Lu-65), which are known to have c-myc gene amplification. In contrast, c-myc protein was undetectable in 1 human tumor (H69) without c-myc gene amplification as well as in human heart tissues.