The following results were obtained by analyzing serum IgG, IgM and IgA of blood from the umbilical cord chiefly of small-for-date infants (term SFD infants): (1) The serum IgG level was higher in the infant's blood than in the mother's blood in the cases of appropriate-for-date infants (term AFD infants). Its fetus/ mother ratio was 1.25 +/- 0.22. (2) The fetus/mother ratios of IgG in term SFD infants and premature infants were lower than in term AFD infants. (3) The placental transmission of IgG increased as the gestation weeks went by, and its fetus/mother ratio reached 1.0 by the 38th week of pregnancy. (4) The serum IgM level was lower in term SFD infants than in term AFD infants. (5) Term SFD infants showed no difference of serum IgA level from that of term AFD infants.