The use of organothallium compounds for protein/macromolecule modification and as probes for n.m.r. and fluorescence is introduced. Lactate dehydrogenase from a number of species was rapidly and specifically inhibited by o-carboxyphenylthallium(III) bistrifluoroacetate and p-methylphenylthallium(III) bistrifluoroacetate. Inhibition of rabbit muscle lactate dehydrogenase by o-carboxyphenylthallium(III) bistrifluoroacetate was time-dependent and not reversible by gel filtration. A small degree of re-activation was possible by incubation with dithiothreitol. The time course of the inactivation kinetics showed two phases, only the first, and faster, of which was efficiently prevented by the presence of cofactor, NADH. Inhibition rates depended on the structure of the thallium reagent, its concentration and the temperature. No significant inhibition was found by thallous acetate or thallic trifluoroacetate. Saturation kinetics were observed for the inhibition by o-carboxyphenylthallium(III) bistrifluoroacetate of the pig heart enzyme. The possibilities of various cross-linking activities of these reagents are addressed. Mechanisms of the inhibition are discussed.