Immunoglobulins IgG, IgA and IgM were studied in serum and cerebrospinal fluid (CSF) samples from 36 newborn infants in the second day of life. According to parameters previously established in the same laboratory for normal newborn infants, serum proteins were normal in all samples and CSF composition was normal as to blood pigments, cells, total proteins and protein fractions in all samples. Immunoglobulins concentrations were determined by two methods: radial immunodiffusion (RID) and nephelometry. Concentrations found (mean +/- standard deviation) were as follows (mg/dl). Serum: through the RID method IgG 1346 +/- 299.1, IgM 10.4 +/- 2.26, IgA 1.1 +/- 1.06; through nephelometry IgG 1372 +/- 319.7, IgM 10.6 +/- 4.10, IgA 1.5 +/- 0.31. Cerebrospinal fluid: IgA and IgM were not detected by the methods employed; IgG was detected by both methods, and values were 11.1 +/- 2.52 by RID and 9.6 +/- 7.04 by nephelometry. IgG relations and IgG index were also evaluated; values of the IgG index (0.5 +/- 0.14) are emphasized. Considerations are made in order to point out data obtained as to characterize normal values for immunoglobulins in the cerebrospinal fluid of normal newborn infants.