In 20 patients, we investigated the effect of interleukin-2 (IL-2) treatment during adoptive immunotherapy for various cancers on circulating levels of: thiamin; biotin; folate; pantothenate; riboflavin; nicotinate; vitamins A, B6, B12 and E; carotenes; free and total cholines; inositol; and free and total carnitines. Of the above micronutrients, only vitamins A, B6, B12, inositol, carotenes and folate varied markedly from normal levels (pre IL-2 exposure) to abnormal levels (post IL-2). Following IL-2 exposure, every patient's B12 level was significantly elevated; 50% of the levels were abnormally increased above 1000 pg/ml. Extreme significant elevations of inositol were also seen in 90% of the patients. In contrast, IL-2 exposure depressed normal vitamins A, B6, carotene, and folate levels to subnormal; 90% of the patients became B6 hypovitaminemic; 60% for vitamin A, 80% for carotene, and 45% for folate. Other micronutrients tested showed no clear deviations from normal levels post IL-2 exposure. Some reasons for micronutrient variations are discussed.