He used many different kinds of people (I'm not trying to be offensive by saying kinds, sorry if I offend anyone). He basically used anyone with out of the ordinary characteristics. Some examples were dwarfs, giants, hanchbacks, and twins. He was mainly interested in twins. They were the perfect research specimens for him. Fraternal twins had similar genes, and identical twins had the exact same ones. Thus, he could see how the twins' bodies reacted to different things. He conducted many tests on these twins who were usually little children. He would often to X-rays and blood tests. If the child was too young, he would draw blood from the neck instead of arm, which would be incredibly painful. Mengele was rumored to have killed some children this way because they bled to death. He also was fascinated with the possibility of trying to change their hair and eye color. He would use eye drops or shots with dye to try to change their eye color. Imagine how painful getting a shot in your eye would be.