CO transfer factor for lungs (TLCO) was measured by a steady state method in 83 subjects (haemoglobin concentration from 60 to 270 g.l-1). TLCO was related with height (T), age (A) and with logarithm of haemoglobin concentration for a non negligeable part : TLCO = 15.45 log Hb + 33.1 T -- 0.06 A -- 51.8. The interest of this relation is discussed.