The lumbar spinal fluid of 70 infants aged 238-461 days after conception was examined for total protein with the Cu-Folin-differential test, and for variation of single CSF proteins as a function of developmental age using agar gel electrophoresis. In the early stages of development all CSF fractions as well as total protein are increased compared with adult persons. The absolute amounts decrease after birth more or less rapidly (partly exponentially) and reach a minimum between the 52nd and 57th week of development (prealbumin and beta 1-globulin somewhat earlier, albumin somewhat later). Except for beta 1-globulin this minimum is always lower than the normal adult values. There seems to be a linear correlation between the logarithm of the total protein (also of certain single proteins) and the gestational age over a wide range of development. The behaviour of the haptoglobin and immunoglobulin fractions demonstrates especially well that many independent factors may simultaneously influence the concentration of the CSF proteins (e.g. plasma content of proteins, permeability of the blood/brain or blood/CSF barrier) and that these factors themselves depend on both the developmental state (gestational age) and chronological age (birth). This study, in agreement with the results of other authors suggests that in human neonates the reference range of those CSF parameters which depend on barrier conditions should be related to the gestational age rather than to the chronological age.