Two vaccine trials were conducted with low- and high-dose purified Twen-ether-treated SIVmac adsorbed onto aluminum hydroxide using rhesus macaques. In the first experiment 7 macaques were immunized with a total amount of 560 micrograms protein and 3 animals served as controls. After the immunization period the vaccinees exhibited ELISA titers up to 1:1280 and 5 immunized animals showed an antigen-specific proliferative response. After the virus challenge the 3 control animals and 3 vaccinees became infected. Four of the infected animals developed a cytotoxic T-cell response beginning 8 weeks postchallenge. The 4 protected animals were rechallenged 16 weeks later and all became infected. For the high-dose experiment 5 immunized animals receiving 2 mg of antigen and 2 control animals were used. The ELISA titers of the vaccinees reached 1:20480 and 4 animals exhibited an antigen-specific proliferative response. In response to virus challenge the 2 control and 1 immunized animal became infected. From these data it can be concluded that the high-dose immunization scheme elicited higher antibody titers and increased the fraction of protected animals.