Cell surface thiols are required for a line of cloned murine natural killer lymphocytes to bind to and lyse tumor target cells. These lymphocytes neither bound to nor killed YAC-1 or G1Tc cells when the effector lymphocyte cell surface thiols were covalently coupled with the non-penetrating reagent, monobromotrimethylammoniobimane (qBBr). A limited number of thiol-bearing proteins were identified by gel electrophoresis on the cell surface using the fluorescence of the group that remains associated with the sulfur molecule. These results indicate that either one or more of these reactive proteins or different cell surface thiol-bearing molecules present at low frequencies are crucial to lymphocyte binding and killing. In contrast, we found little evidence that intracellular thiols are required for natural killer cell activity. Killing was relatively unimpaired when over 90% of lymphocyte glutathione was depleted with DL buthionine-S,R-Sulfoximine (BSO). Blocking the intracellular or the extracellular thiols of tumor targets had no effect on their ability to be lysed. Based on these data, we suggest that infrequently expressed extracellular thiols are required either for the conformation or for the disulfide crosslinking of proteins that participate in lymphocyte-mediated cytolysis.