The inhibitory effects of the cationic triarylmethane (TAM + ) dyes, pararosaniline (PR + ), malachite green (MG + ), and methyl green (MeG + ) on human plasma cholinesterase (BChE) were studied at 25°C in 100mM Mops, pH 8.0, with butyrylthiocholine as substrate. PR + and MG + caused linear mixed inhibition of enzyme activity. The respective inhibitory parameters were K i =1.9±0.23μM, α=13±48, β=0 and K i =0.28±0.037μM, α=23±7.4, β=0. MeG + acted as a competitive inhibitor with K i =0.12±0.017μM (α, ∞, β, not applicable). The K i values were within the same range reported for a number of ChE inhibitors including propidium ion, donepezil, and the phenothiazines, suggesting that TAM + s are active site ligands. On the other hand, the α values failed to correlate with values previously reported for a number of ChE inhibitors. It appears that mixed inhibition is the combined result of more than one type of binding and S–I interference. The impact of ligands at the choline-specific and peripheral anionic sites (or, possibly, accessory structural domains) on BChE activity needs to be studied in further detail.