BUENOS AIRES — DNA tests have identified a person taken from his mother as a baby by Argentina’s former dictatorship, a rights group said, bringing the number of such cases to 128.

The group, Grandmothers of the Plaza de Mayo, said on Friday that the person, who was identified only as Marcos, is the biological son of a woman named Rosario del Carmen Ramos.

In 1976, Ms. Ramos was kidnapped by former military and police officials in the northwestern province of Tucumán, along with her son, who was 5 months old at the time, and one of his half brothers. Ms. Ramos was never found, and the two boys were taken to separate homes.

Marcos, now 42, found out about his true identity on Thursday night and met with family members. The announcement was made at an emotional news conference attended by two of his half brothers.