The Navajo are known to be at high risk for hepatitis A virus (HAV) infection. This study investigated the safety and immunogenicity of an investigational, alum-adjuvanted, formalin-inactivated HAV vaccine (VAQTA) developed by Merck Research Laboratories in Navajo children. One hundred two of 212 children, ages 4 to 12 years, were HAV-seronegative (<10 mIU/ml by an enhanced sensitivity modification of the HAVAB*; Abbott). Ninety of these children received the HAV vaccine. Study participants were given vaccines containing various viral protein concentrations: Group A (n= 18), 6 units; Group B (n= 36), 13 units; and Group C (n= 36), 25 units HAV protein (1 unit ± 1 ng viral protein antigen). Three-dose (0, 8, 24 weeks) and two-dose (0, 24 weeks) regimens were compared in subgroups within B and C. The vaccine was well-tolerated and there were no serious adverse reactions; no vaccinee developed hepatitis A. After 1 dose 82 to 100% of children seroconverted (≥10 mIU/ml, modified HAVAB*; Abbott) and 100% seroconverted after 2 doses. After 1 dose the geometric mean titer for antibody was: Group A, 22 mIU/ml; Group B, 18 mIU/ml; and Group C, 38 mIU/ml. After 3 doses geometric mean titers increased to 10 106 mIU/ml in Group A, 7258 mIU/ml in Group B and 11 856 mIU/ml in Group C. Further field studies are indicated to evaluate its use in high risk populations, such as the Navajo.