We developed a simple, inexpensive, rapid assay for the detection of antibodies to simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) in serum. The immunoassay uses inactivated SIV and HIV-1 gp41 transmembrane recombinant protein as antigenic adsorbents on a nitrocellulose filter membrane. Diluted serum, with the addition of Protein-A-Gold, is gravity-filtered through the filter membrane, blocked, and buffer-washed. Antibodies to HIV or SIV or both in serum bind to the appropriate antigen, and the resulting antigen-antibody complex reacts with Protein-A-Gold to produce a readable pink color. Field evaluation of the test on 30 human and 70 nonhuman primate sera in Kenya and Zaire indicated that the test had at least 93 and 90% correlation with Western blot sensitivity and specificity respectively. Prior refrigeration of the test kit and incubation of sera during testing were not required. This result indicates that the test may be a rapid, economical, and simple test for detecting HIV, SIV, or both in serum. This immunoassay can be useful for carrying out HIV and SIV serosurveys in countries with limited or no laboratory facilities.