Twelve cases of neuroblastoma (NB) (7 boys and 5 girls) and 4 cases of primitive peripheral neuroectodermal tumor (PNET) (3 boys and 1 girl) were investigated for the presence of apoptosis and retinoic acid receptor (RAR) by immunhistochemical method. The apoptotic index in NB was zero or 1% in 8 children and relatively low (2-4.8%) in the other 4 cases, while it was higher (4.1-10.5%) in PNET. The RAR index determined by immunoperoxidase reaction in NB was zero or 3% in 5 cases and 9-34% in 7 children. RAR index in PNET was 16-68% in all the 4 cases. Good correlation (r = .47 according to Pearson-Bravis) was found between the number of RAR and spontaneous apoptosis. These results suggest that the RAR index in untreated NB and PNET shows great individual variation since its determination is necessary for the evaluation of the efficacy of retinoic acid treatment.