Several human neuroblastoma cell lines express transcripts of the proopiomelanocortin (POMC) gene of 0.8 kb and 9.5 kb, thus different from the characteristic pituitary 1.2 kb POMC mRNA. The expression of POMC mRNA decreases when the cell lines are differentiated with retinoic acid or alpha-difluoromethylornithine. Since both POMC and MYCN genes are present on the same chromosome segment (2p23), it is possible that the 2 genes are coamplified and coexpressed. Alternatively the expression of opiate peptides by neuroblastoma cells might act as an autocrine factor or it may modulate the action of tumour infiltrating lymphocytes.