The BB fraction of serum creatine kinase was estimated in 3 groups of newborn : 30 healthy newborn at term, 20 newborn ill but without cerebral distress, 17 newborn with acute perinatal or foetal distress. In the first two groups, BB fraction was negligeable or nil, in respectively 26 and 16 cases, and between 1 and 4% in 4 cases. In the third group, it was greater than 4% in 10 cases, nil in 6 and 4% in one. The lability of this isoenzyme over periods of time, and in relation to temperature, requires the use of clearly defined techniques of sampling, storage and estimation, which may explain the false negatives seen. The determination of this isoenzyme, virtually exclusively cerebral, would thus appear to be a reliable biological indicator of neonatal cerebral distress.