Two cationic phenoxazine dyes, meldola blue (MB) and nile blue (NB), and the structurally related phenothiazine, methylene blue (MethB), were found to act as complex inhibitors of human plasma cholinesterase (butyrylcholinesterase, BChE). Studied at 25 degrees C, in 100mM MOPS buffer (pH 8.0), with butyrylthiocholine as substrate, the kinetic pattern of inhibition indicated cooperative I binding at 2 sites. Intrinsic K' values ( identical with[I](0.5)(2) extrapolated to [S]=0) for MB, NB and MethB were 0.64+/-0.05, 0.085+/-0.026 and 0.42+/-0.04 microM, respectively. Under the same experimental conditions the dyes acted as single-occupancy, hyperbolic-mixed inhibitors of electric eel acetylcholinesterase (AChE), with K(i)=0.035+/-0.010, 0.026+/-0.0034 and 0.017+/-0.0063 microM (for MB, NB, MethB); alpha (coefficient of competitive interaction)=1.8-2.4 and beta (coefficient of noncompetitive interaction)=0.15-0.28. The complexity of the BChE inhibitory effect of phenoxazine/phenothiazine dyes contrasted with that of conventional ChE inhibitors which cause single-occupancy (n=1), competitive or mixed inhibition in both AChE and BChE and signaled novel modes of ligand interaction at (or remote from) the active site gorge of the latter enzyme.