Features of the apoptotic response evident in glucocorticoid-treated thymocytes are not uniformly observed in cell lines exposed to anticancer drugs. The significance of such variation has been assessed by monitoring molecular and cellular processes induced by etoposide (VP-16) in the human lymphoblastoid T-cell lines CCRF-CEM (CEM) and MOLT-4 contrasted, where appropriate, with those induced by necrotizing injury. Cytotoxic concentrations of the drug were determined to be 5-100 microM on the basis of tetrazolium reduction assay. The two lines were equally sensitive to VP-16; no difference in concentration of drug which inhibited cell growth by 50% with respect to control (i.e., drug free) cultures was apparent irrespective of exposure times from 3-72 h. DNA strand breaks were evident in both populations within 3 h of exposure to VP-16. Morphological change, assessed microscopically, involving nuclear condensation and cell shrinkage was qualitatively and quantitatively similar in VP-16-treated CEM and MOLT-4 cells. Flow cytometric analysis indicated that the G2/M fraction of the randomly dividing MOLT-4 population was approximately one-third that of CEM cells, but each line exhibited a decrease in this fraction 3-6 h after treatment. Despite these similarities, marked differences in the response to VP-16 were evident in the two populations. Internucleosomal fragmentation, detected electrophoretically 3 h after treatment in DNA isolated from CEM cells, was not detected under any condition in MOLT-4 DNA. Apoptotic bodies, also evident within 3 h of VP-16 treatment of CEM cells, were not readily apparent in MOLT-4 cells under the same conditions. Treatment causing necrosis resulted in trypan blue uptake within 1 h in a similar high proportion of cells from both lines. Exposure to VP-16 resulted in such a loss of membrane integrity by 6 h in CEM cells, while change in this parameter occurred only after 24 h in the case of MOLT-4 cells. The findings indicate a wide scope of difference in apoptotic response and suggest delayed loss of membrane permeability, rather than DNA fragmentation, as the clearest indicator of programmed cell death in these populations.