Polymerase chain reaction amplification (PCR) is a recently described technique that allows for the amplification of a given sequence of DNA. It can be used to reliably amplify sequences of up to 3 kb within hours. The amplified sequence can then be recognized by hybridization with a specific probe after transfer onto nitrocellulose or nylon paper. We used PCR to recognize human B-lymphotropic virus (HBLV or HHV-6) specific sequences in various tumors as well as in the blood of patients with AIDS. Sixty-three specimens of DNA extracted from peripheral blood of patients with AIDS as well as DNA extracted from various lymphoproliferative disorders were analysed; 52 out of 63 (83%) patients with AIDS were found to have amplification of the HHV-6 specific sequence; 2 out of the 63 (3%) had equivocal amplification and 9 (14%) were found to be negative. Twenty out of 23 tumors were found to have amplified HBLV-specific sequences. Only one of these tumors was positive by Southern hybridization on restriction enzyme digested genomic DNA. In situ hybridization of clinical specimens using radiolabelled RNA probes or hapten-labelled DNA probes was used to detect the presence of HBLV in tumors. Three tumors of B cell origin were found to be positive for HBLV.