Spontaneous lymphoblastoid cell lines (LCLs) were established from the peripheral blood of 10 human immunodeficiency virus (HIV)-seropositive patients in order to investigate whether or not progression of the cells towards a malignant state could be traced. The LCLs studied displayed no differences in their surface phenotype, karyotype, and tumorigenicity in nude mice as compared with a wide panel of control LCLs. Furthermore, no c-myc rearrangement could be detected in any of the LCLs. However, 4 of the 10 LCLs derived from HIV-seropositive patients formed colonies in agar with a cloning efficiency of 0.1-0.9%. This percentage was much lower than that of a control neoplastic B cell line (50%), but consistently higher than that observed for a battery of spontaneous LCLs. The cells of a number of sublines that were derived from the agar colonies expressed new activation markers (CD10 and Bac-1) but did not induce tumors in nude mice or display chromosomal abnormalities. These sublines might comprise cells that have progressed towards a more markedly transformed state.