Spontaneous lymphoblastoid cell lines (LCLs) were established from the peripheral blood of ten human immunodeficiency virus (HIV)-seropositive patients in order to investigate whether or not a progression of the cells toward a malignant state could be traced. The LCLs studied displayed no differences in their surface phenotype, karyotype, or tumorigenicity in nude mice when compared with a wide panel of control LCLs. However, four of the ten LCLs derived from HIV-seropositive patients formed colonies in agar with a cloning efficiency (0.1 to 0.9%) that was much lower than that of a control neoplastic B cell line (50%). Some sublines that were derived form the agar colonies expressed new activation markers (CD10 and Bac-1) but did not produce tumors in nude mice or display chromosomal abnormalities. These sublines might comprise cells that have progressed toward a more transformed state.