The usefulness of the gluconeogenic key enzyme fructose 1,6 bisphosphatase (FBPase), which is localized exclusively in the proximal nephron segment, as a marker compound to monitor injury of the proximal nephron segment during nephrotoxic therapy, was tested in a collective model of male patients treated for testicular cancer. These patients with normal kidney function were submitted to therapy with the nephrotoxic chemotherapeutics carboplatinum and a combination of cisplatinum, etoposide, bleomycin and ifosfamide. The release of FBPase activities into the urine was monitored during the initial two treatments over a period of eight days. The urinary enzyme activities measured were compared to the excretion of the "proximal tubular injury markers" N-acetyl-beta-D-glucosaminidase (NAG) and alpha 1-microglobulin (alpha 1m). The presence of glomerular damage was determined by measurement of urinary excretion rates of albumin (ALB) and IgG. In addition, protein excretion patterns following chemotherapy were monitored. The combined administration of cisplatin, etoposide and ifosfamide resulted in a pronounced proximal tubular injury as shown by the release of FBPase into the urine. This is substantiated by simultaneously increased excretion rates for NAG and alpha 1m. Proximal tubular toxicity was found to be less severe when cisplatin was combined with etoposide and bleomycin and was nearly absent following carboplatinum monotherapy. Carboplatinum only affected glomerular function and resulted in an elevated ALB and IgG excretion. From this model investigation it can be delineated that determination of urinary FBPase activities ensures a sensitive and reliable identification of proximal nephron damage.