1. Measurements of transfer factor and sub-maximal exercise ventilation and cardiac frequency have been made on twenty women with iron-deficiency anaemia (Hb 8–9 g/100 ml) before and after ‘treatment’ with iron or placebo tablets and on control subjects. 2. The exercise ventilation, cardiac frequency and oxygen uptake were independent of haemoglobin concentration but the transfer factor was lower in the test than in control subjects and was increased by iron but not by placebo treatment. The results support the validity of the reaction-rate data for carbon monoxide with oxyhaemoglobin of Roughton & Forster (1957) despite evidence to the contrary from other studies. 3. In interpretation of sub-maximal exercise ventilation and cardiac frequency in iron-deficiency anaemia no allowance need be made for variation in haemoglobin concentration in the range 8–15 g/100 ml. For transfer factor a correction should be made by using a variant of the relationship of Roughton & Forster (1957).