The mechanism of clearance of human immunodeficiency virus-1 (HIV-1) viraemia is not fully understood. In two patients with acute HIV-1 infection, initial high titres of free infectious virus in plasma declined rapidly to undetectable levels within 4-8 weeks, an event that was coincident with seroconversion. Neutralising antibodies directed against the first autologous isolates taken during the viraemic periods could not be detected in either patient around the time of disappearance of plasma virus. In the absence of functional neutralising antibody, it is unlikely that humoral factors are responsible for the suppression of primary viraemia in early HIV infection.