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In another publication (1), the molecular size of antibodies formed in response to immunization with typhoid-paratyphoid (TPT) vaccine was studied in adult humans. In this report premature infants, immunized shortly after birth, were studied in a similar manner in order to compare this aspect of the immune response in infants and adults. It has been shown (2) that the initial antibodies produced in response to TPT immunization are 19S macroglobulins. The sequence of formation of macroglobulin antibody, followed by 7S antibody, has also been observed by Taliaferro, Talmadge, Freter and Stelos (3-7), who showed that in rabbits immunized with sheep or beef red cells the first antibodies formed had an electrophoretic mobility of the y-1 type and had a sedimentation coefficient of 18S. These were followed, in time, by 6S antibodies associated with the y-2 globulins. Molecules of antibodies of different size in rabbits, produced in response to single protein antigens, have recently been reported (8).We were also interested in determining the effect of maternally-transferred antibody on the formation by infants of agglutinins to TPT antigens. In addition, the effect of sulfhydryl compounds on the macroglobulin antibodies produced by the premature infant was investigated. It has been suggested that the agglutinin activity of 19S typhoid H antibody of the neonatal infant differs qualitatively from that of the adult after treatment with 2-mercaptoethanol (9). t Fellow, The Helen Hay Whitney Foundation. MATERIALS AND METHODSForty-two premature infants were studied. Preimmunization blood was obtained from the umbilical cord or from the infant immediately before immunization. In a number of cases both were obtained. Infants were immunized by intramuscular injection into the anterior thigh of 0.25 ml of a TPT A and B vaccine (Lederle) containing 1 billion organisms of Salmonella typhi per ml and 250 million organisms of S. paratyphi A and B per ml. Thirty-one infants received three injections 1 week apart; eleven received only one injection. The first injection was given within 48 hours after birth and in most instances within the first 24 hours. The first blood sample was obtained 1 week after the third injection in most cases. Additional blood samples were obtained before and after booster injections given at varying times after the initial course of immunization.Fourteen older children, aged 9 months to 8 years, hospitalized for orthopedic rehabilitation or convalescing from primary pulmonary tuberculosis were immunized with three injections of 0.5 ml of TPT vaccine given intramuscularly, 1 week apart. Sera were obtained before immunization and at various times after the third inj ection.Blood samples were allowed to clot at 370 C and the sera removed. Each serum was diluted with an equal volume of distilled water and dialyzed for 2 days against 0.01 M phosphate buffer at pH 8.5. After dialysis, sera were stored at -20°C. The dialyzed sera, usually 1 to 3 ml, were fractionated by column chromatography on diethylaminoethyl ...