To study the clonal events occurring during ontogeny of the humoral immune system, we evaluated plasma immunoglobulin (Ig) production in term newborns and young children by high-resolution two-dimensional gel electrophoresis. The clonality pattern of Ig light (L) chains from healthy newborns (n = 19) was similar to that observed on protein maps of their mothers or of normal adults (n > 100), that is, rare distinguishable small spots in a cloud-like large band of indiscrete Ig L chain spots (polyclonal pattern; maternal Igs). Analysis of plasma samples obtained from infants between 1 month and 5 years of age (n = 55) revealed discrete but evident alterations of the clonality of Ig production. Between the 2nd and 4th months of life, transient attenuation of the "polyclonal background" was observed in association with the appearance of an increasing number of well-resolved Ig L chain spots (corresponding to plasma Ig concentrations between 0.5 and 1 g/L per spot). This "restricted" clonal pattern was progressively less apparent on protein maps of infants older than 2 year and evolved towards a "normal" adult polyclonal pattern at the age of 5. These results suggest that the development of the B-cell clones is heterogeneous, either through limited outgrowth of precursor cells or through selective antigenic pressures.