Unheated human serum does not lyse the AIDS-associated retrovirus (ARV), change the density of the virus, or suppress its ability to infect human peripheral mononuclear cells. The results indicate that ARV and the human oncovirus HTLV-I, unlike other animal retroviruses, are resistant to the effect of human serum. These two human viruses coming from different retrovirus subfamilies may be pathogenic because of this lack of sensitivity to human complement components.