In addition to the two forms of haemoglobin previously recognized in the blood of the foetus and newborn, a third form has been detected by following the rate of reaction with sodium hydroxide. This component appeared in 33 of the 66 samples studied but could not be detected in the remainder. No explanation could be found for this anomaly. Confirmation was obtained of the presence of a second form of haemoglobin in adult blood. This was shown to differ from the most refractory fraction of foetal blood. It is suggested that the older terminology is no longer sufficiently definitive and an alternative scheme is proposed.