We studied ten patients with various types of cancer who were being treated with Interleukin-2 (IL-2) and lymphokine activated killer cells (LAK). All patients developed a reticulocytopenic, normochromic, normocytic anemia. We noted some variability but no significant suppression of circulating erythroid progenitors. The levels of erythropoietin were lower than expected for the hemoglobin/hematocrit values. We could not detect Interferon or Tumor Necrosis Factor (TNF) in the serum of these patients; however, the supernatant of LAK cells did contain Interferon and TNF which could be neutralized with appropriate antibodies. These results suggest that the etiology of this anemia is multi-factorial. Administration of recombinant erythropoietin (Ep) may be of benefit in some of these patients.