We determined O6-alkylguanine-DNA alkyltransferase (AGT) activity in the peripheral blood lymphocytes (PBLs) of normal controls and patients with Hodgkin's disease or non-Hodgkin's lymphoma and compared these values with those of Epstein-Barr virus (EBV)-transformed cell lines prepared from the same PBL samples. PBLs have an AGT level characteristic of the individual from whom the cells were obtained. The AGT activity of lymphoblastoid cell lines obtained from a control group of PBLs was significantly correlated with the activity of the PBLs from which they were derived (r = 0.742). There was no significant correlation between PBLs and EBV-transformed lines derived from these PBLs in Hodgkin's disease/non-Hodgkin's lymphoma patients (r = 0.407, -0.225, and 0.270 for patients prior to, during, or after therapy, respectively). The lack of significant correlation between lines and PBLs was not due to random fluctuations in AGT activity, because multiple lines prepared from the same PBL sample were found to be highly correlated in AGT activity. In order to account for these results, we suppose that PBLs from a given individual are a heterogeneous population with respect to AGT activity. In normal individuals, the AGT activity of early passages of the multi-clonal EBV-transformed cell lines reflect the AGT activity of the PBLs from which they were derived. Malignancy and/or treatment with chemotherapeutic agents may selectively affect those lymphocytes which are targets for EBV-transformation so that the resultant cell line is no longer representative (with respect to AGT activity) of the total PBL population. Long-term culture of lymphoblastoid cell lines results in changes in AGT activity in some but not all cell lines suggesting that with time in culture, subsets with different AGT activities may be selected. There appears to be no growth advantage of low AGT activity and only rarely have we obtained lines with no measurable AGT activity, even after long periods in culture.