Ten patients with disseminated testicular cancer were studied during 3 cycles of 20 mg/m2/day x 5 d CDDP combined with bleomycin and vinblastine or etoposide. Five of the patients (= group A) received 0.4 mmol KCl/kg/day and 0.3 mmol MgCl2/kg/day; the other five (= group B) received only KCl intravenously during the CDDP infusions. In group A no decrease of serum and red blood cell Mg concentration was noted. In group B the serum Mg concentration decreased from 0.84 +/- 0.04 mmol/l to 0.57 +/- 0.10 (p less than 0.001) on day 47, to 0.64 +/- 0.08 (p less than 0.005) on day 50 and to 0.72 +/- 0.02 (p less than 0.05) on day 57. The Mg concentration in the red blood cells decreased, but not significantly in group B. Furthermore, the fractional calcium excretion in group B was significantly lower than in group A during the CDDP infusions.