Increased N-myc (now designated NMYC in human gene nomenclature) gene expression has been detected at the transcriptional level in certain types of neoplasms. As yet, the N-myc gene product has not been identified. To detect and characterize the N-myc gene product, we have developed monoclonal antibodies against the putative N-myc gene product made in Escherichia coli as a fusion protein. The antibodies that recognize the N-myc-specific regions were selected on the basis of their reactivities to different portions of the fusion protein. These monoclonal antibodies detect a pair of closely migrating polypeptides of 60 and 63 kDa in nuclear fractions of human neuroblastoma cells. The relative levels of the polypeptides are roughly proportional to the level of N-myc transcripts present in a panel of neuroblastoma lines. These two polypeptides have a half-life of approximately equal to 35 min, and they are indistinguishable from each other by their epitopic profiles.