Serum immunoerythropoietin (SIE) levels were studied of 25 randomly chosen cancer patients undergoing cisplatin-based chemotherapy and ten head and neck cancer patients who were studied prospectively before and during cisplatin-based therapy. The SIE levels were determined by standard radioimmunoassay, and the results were interpreted relative to erythropoietin levels and hematocrits of 17 aplastic or nutritionally anemic patients who were believed to have a normal erythropoietin response. Of the 25 randomly chosen patients, SIE levels were inappropriately low in four patients. In this population, there was a greater likelihood of erythropoietin deficiency in the patients with a hematocrit less than 30% compared with those with higher values (P less than 0.001), although there was no correlation between SIE level and the amount of cisplatin these patients received or their degree of renal impairment. Of the ten head and neck cancer patients, five were found to have inappropriately low SIE levels before therapy, and two additional patients had a decrease of SIE levels during therapy, in one patient to an abnormally low level. The anemia associated with malignancy was concluded to be in part associated with a relative erythropoietin deficiency, and in certain individuals, cisplatin therapy may contribute to that deficiency.