Since 1987 the Expanded Program on Immunization in Indonesia started to administer DPT and Polio vaccines in 2 month infants. The titer of maternal antibodies against diphtheria and tetanus in new born babies was high. The objective of this study is to determine the effectiveness of diphtheriae and tetanus toxoid of DPT vaccine administered in 2 months infant. The study was carried out in Tresnawati hospital in Yogyakarta. Samples of 83 of 2 month infants and 36 of 3 month infants were given 3 dozes of DPT vaccines. Blood was taken before and after immunization of DPT 1, 2 and 3. Antibodies against diphtheriae and tetanus were measured by passive haemaglutination test. The maternal antibody against diphtheriae was effected immune response after DPT 1 and DPT 2 and for tetanus after DPT 1. The percentage of protected infants and mean titer of tetanus in group of 2 month infants was lower than control but there was no significant difference between the 2 groups. They were protected 100% after DPT 2, and 3. The percentage of protected infants against diphtheriae before and after DPT 1, 2, 3 in the group of 2 month infants were 98,7%, 36,4%, 58,9 % and 83,6 % compared with 61,5%, 42,3%, 88,5% and 100%. There was significant difference between the 2 groups. There was a different effect of maternal antibodies between diphtheriae and tetanus. The high titer of diphtheriae antibody was caused by natural infection while the high titer tetanus was caused by immunization of TT given to their mothers during pregnancy. This study suggest the importance of avoiding drop out of DPT 3.