Ten patients were treated with 10 g essential amino acids per day orally; 9 patients received 9.5 g of a mixture of essential amino acids (Lys, Thr, Try, His, Tyr) and ketoanalogues of Ile, Leu, Phe, Val, Met per day and a control group of 11 patients received no supplementation. All patients were on a liberal food intake amounting to 1g protein/kg body weight and 31 kcal/kg body weight daily. Before and three months after the beginning of supplementation the following parameters were measured: serum concentrations of albumin, transferrin, urea, creatinine, blood haemoglobin content and haematocrit, activities of the enzymes delta-aminolevulinic acid dehydrase and porpho-bilinogen desaminase, and globin synthesis in peripheral red blood cells. After treatment with either essential amino acids or keto acids a significant stimulation of globin synthesis occurred. None of the other parameters was altered. It is concluded that in well-nourished patients supplements of essential amino acids or keto acids are ineffective.