Cis-dichlorodiammineplatinum (CDDP) and VM26, both of which have been proven efficient in treating neuroblastoma, were combined in a sequential schedule and administered to 22 children with disseminated neuroblastomas resistant to treatment with cyclophosphamide and doxorubicin (Adriamycin). During this same study, 14 children were prospectively evaluated for the effect of CDDP on magnesium metabolism and the effect of the induced hypomagnesemia on parathyroid function. Complete or partial tumor responses were achieved in six and nine cases. Respectively and were of prolonged duration (longer than six months) in eight of the 15 responding. It was also shown that CDDP-induced hypomagnesemia is the result of excessive renal loss and is severe enough to interfere with the normal parathormone response to hypocalcemia.