Combination of dibutyryl adenosine 3', 5'-cyclic monophosphate or prostaglandin E1 (PGE1) and papaverine effectively induced differentiation of neuroblastoma in mice. Two cases of human neuroblastoma with stage III and IV were administered intraaortic PGE1 infusion combined with oral papaverine and conventional chemotherapy. There were no noticeable side effects and the treatment was effective in decreasing tumor size and promoting tumor maturation in the infused area. However, distant osseous metastases were developed in both cases, during and after the PGE1 administration. They survived 30 and 17 months, respectively, from the initiation of therapy. (Jpn J Cancer Chemother 10(9): 1936-1943, 1983) These results prompted us to study the metastatic potentials of neuroblastoma. In vitro studies demonstrated that cultured human neuroblastoma cells (NB-1, GOTO, SK-N-DZ, SJ-N-KP, SJ-N-CG and SK-N-FI) aggregate human platelets with maximum aggregation ranging from 28% to 51%. Addition of PGE1 or PGD2 to PRP effectively inhibited the tumor-cell-platelet interaction, with IC50 approximately 100 nM for PGE1 and 10 nM for PGD2, respectively. In addition, 50 microM PGE1 or PGD2, 5 microM PGI2 reversed neuroblastoma-induced platelet aggregation in 4 out of 5 cell lines were studied. These findings indicate a the possible role of PGs in effective inhibition of neuroblastoma metastases in vivo. However, two cell lines (SK-N-DZ and SJ-N-CG), which had been exposed to 8.5 microM PGE1 or PGD2 for 90 min and 72 hr, respectively, retained the platelet aggregating activity which was not significantly different from that of untreated cells. We conclude that clinical application of intraaortic PGE1 in the treatment of advanced neuroblastoma has advantage in potentiation of tumor cell kill and in inducing maturation. Administered PGE1 may exert its action in two ways: in preventing tumor metastasis or possibly in enhancing the metastatic potential of neuroblastoma cells. Further refinement of these modalities including other PGs such as PGD2 or PGI2 and more detailed studies on optimal PG administration to prevent metastasis should be evaluated in future.