Oximes are cholinesterase reactivators of use in poisoning with organophosphorus ester enzyme inhibitors. Pralidoxime (PRX) is the oxime used in the United States. Clinical experience with pralidoxime (and other oximes) is disappointing and the routine use has been questioned. Furthermore oximes are not equally effective against all existent enzyme inhibitors. There is a clear demand for 'broad spectrum' cholinesterase reactivators with a higher efficacy than those clinically available. To meet this need over the years new reactivators of cholinesterase of potential clinical utility have been developed. The purpose of the study was to quantify 'in vitro' the extent of protection conferred by available (pralidoxime and methoxime) and experimental (K-27, K-33 and K-48) oximes, using methyl-paraoxon (methyl-POX) as an esterase inhibitor and to compare the results with those previously obtained using paraoxon (POX) as an inhibitor. Red blood cell (RBC) acetylcholinesterase (AChE) activities in whole blood were measured photometrically in the presence of different methyl-POX concentrations and IC(50) values calculated. Determinations were repeated in the presence of increasing oxime concentrations. The IC(50) of methyl-POX (59 nm) increased with the oxime concentration in a linear manner. The calculated IC(50) values were plotted against the oxime concentrations to obtain an IC(50) shift curve. The slope of the shift curve (tg alpha) was used to quantify the magnitude of the protective effect (nm IC(50) increase per microm reactivator). Based on our determinations the new K-series of reactivators is superior to pralidoxime (tg alpha = 1.9) and methoxime (tg alpha = 0.7), K-27 and K-48 being the outstanding compounds with a tg alpha value of 10 (nm IC(50) increase per microm reactivator), which is approximately five times the reactivator ability of PRX. The tg alpha value determined for K-33 was 6.3. The ranking of reactivator potencies of the examined oximes determined with methyl-POX as an inhibitor (K-27 = K-48 > K-33 > pralidoxime > methoxime) is similar to the ranking previously reported by us using POX as an inhibitor (K-27 > or = K-48 > K-33 > methoxime = pralidoxime). There is an (expected) inverse relationship between the binding constant K and the slope of the IC(50) shift curve (tg alpha) for all oximes examined. K-27 and K-48 (the most protective substances judging by the tg alpha) having the lowest K value (highest affinity). In vivo testing of the new oximes as methyl-paraoxon protective agents is necessary.