In 50 patients with seminoma and in 50 with nonseminomatous germ cell tumors of the testis, serum levels of conventional markers (CEA, AFP, hCG) and ferritin were measured at the time of admission and during management. The conventional markers behaved as reported previously. After orchiectomy, elevated levels of ferritin were found in the presence as well as in the absence of tumor; the extent of these elevations was highly variable. Serial determinations of serum ferritin showed two patterns of variation. First, in patients treated with retroperitoneal lymph node dissection, irradiation, and chemotherapy regimens without platinum, decreasing levels of the conventional markers and serum ferritin were associated with response to therapy and increasing levels with relapse of tumor. Second, in patients treated with chemotherapy regimens containing cis-diamminedichloroplatinum, the conventional markers returned to normal values while the ferritin level doubled or tripled and returned to pretreatment or normal values only several weeks after therapy. Thus, it appears that hyperferritinemia was a sensitive index of platinum toxicity. We conclude that the serum ferritin level has no value in staging after orchiectomy but is a useful index of response to therapy during treatment with retroperitoneal lymph node dissection, irradiation or chemotherapy without platinum or relapse of tumor. During treatment with platinum, elevated levels might be explained as a possible toxic side effect of this drug.