IgM, IgA, and AFP concentrations were determined in a group of 30 umbilical cord sera chosen at random from the cord sera sampled over a period of one year. Because low levels were expected, use was made of sensitive techniques, which were standardized on the basis of international reference preparations. Geometric mean levels were 5.58 mg/l for IgA, 142 mg/l for IgM and 53.6 mg/l for AFP. Statistical analysis of the distinct IgM, IgA, and AFP levels showed no significant correlation with either birth weight or birth length. The difference between the sexes in the levels of these proteins was not significant either. The difference between the IgA concentration in sera from children of primipara and those of multipara mothers was significant. Moreover, a statistically significant negative correlation was found between the IgA and AFP levels. The results are discussed in relation to current knowledge about the ontogeny of humoral immunity in man and a possible biological function of AFP in ontogeny.