Total IgG concentrations, IgG antibody concentrations to pooledEscherichia coliantigens, and IgG anti-E. coliantibody avidity were measured in cord and maternal serum samples collected from 52 mother-infant pairs after premature delivery (mean gestational age 28 wk, range 23-33 wk). The mean IgG anti-E. coliantibody concentration in cord serum (1.86 relative units/mL) was markedly lower than in maternal serum (5.42 relative units/mL) at this gestation (p<0.0001). Cord serum IgG anti-E. coliantibody concentrations correlated closely with maternal IgG anti-E. coliconcentrations when controlled for the effect of gestational age (partial correlation coefficient 0.89;p<0.001) but only weakly with gestational age when controlled for maternal IgG antibody concentrations (partial correlation coefficient 0.23;p=0.06). The mean ratio of cord to maternal IgG anti-E. coliantibody concentrations was considerably lower than the mean ratio for total IgG concentrations (0.34 versus 0.72;p<0.001). The mean avidity of IgG antibody for the pooledE. coliantigens was significantly greater in cord serum than in maternal serum (2.45versus1.99M;p<0.0001). There was a close correlation between cord and maternal antibody avidity (r=0.70;p<0.001), but cord IgG antibody avidity did not correlate with gestational age (r=-0.07;p=0.61), nor with cord IgG anti-E. coliantibody concentrations (r=0.10;p=0.50). Heat stable serum opsonic activity for the pooledE. coliantigens per unit of IgG antibody, measured by augmentation of normal neutrophil iodination responses, was 2-fold greater in cord serum than maternal serum for the 20 serum pairs tested. Heat stable opsonic activity per unit of IgG antibody correlated closely with IgG antibody avidity in cord and maternal sera (r=0.96 and 0.94, respectively). These findings suggest that there is selective transplacental transport of high avidity antibody with enhanced opsonic activity for these antigens early in the 3rd trimester of pregnancy.