Originally developed as antidotes to organophosphorus nerve poisons, the oximes have attracted renewed interest in studies of cellular regulation. In particular, 2,3-butanedione monoxime (BDM) has gained attention as a useful membrane-permeant "chemical phosphatase" for studying roles of protein phosphorylation. It has been proposed that effects of BDM on cardiac muscle tension, action potentials, neuromuscular transmission and ion currents are related to dephosphorylation of substrates as diverse as myofibrils and ion channels. In the present study, voltage-dependent K+ currents in human T lymphocytes were studied using the whole cell patch clamp technique. Preincubating intact cells briefly in 5 mM BDM before recording reduced the K+ current in an irreversible manner, consistent with chemical (phosphatase?) modification of the channels. In contrast, acute BDM treatment produced a rapid, reversible block of K+ current with half block at about 5 mM. Moreover, including adenosine-O-5'-(3-thiotriphosphate) (500 microM) in the patch pipette did not prevent the rapid, reversible block by BDM. Under these conditions, the most likely mechanism was a direct block of channels from the outside. Because similar K+ currents are present in many tissue and cell types, a direct channel block suggests caution in interpreting the effects of oximes as resulting from protein dephosphorylation.