Heterosexual couples of individuals infected by HIV represent a group of high risk of virus infection. We have investigated the presence of fragments of the HIV genome with a new nucleic acid amplification technique (PCR or polymerase chain reaction) in lymphocytes from 33 seronegative couples with anti-HIV antibodies, most of which were heroin addicts. 12 of the 33 couples (36%) had positive PCR for a gag HIV amplimer in the absence of antibodies. 4 of them (12%) also showed reactivity for sequences from an env amplimer. A control group of 7 patients with AIDS and 4 seropositive couples showed reactivity to gag and env in all. It is concluded that silent HIV infection may frequently develop in heterosexual couples of infected individuals.