The kinetics of arginine esterases E-I, E-II and E-III from the venom of Bitis gabonica were investigated. With N alpha-benzoyl-L-arginine ethyl ester as substrate linear competitive inhibition versus L-arginine was observed while ethanol gave rise to S-parabolic I-linear noncompetitive inhibition. Hydrolysis of N alpha-benzoyl-L-arginine-p-nitroanilide was noncompetitively inhibited by p-nitroaniline. Both slopes and intercepts of double reciprocal plots were a linear function of inhibitor concentration. Ethanol gave complex inhibition kinetics which could be interpreted in terms of mixed dead-end and alternate product inhibition (S-parabolic I-hyperbolic noncompetitive inhibition). These results imply an ordered uni-bi as the minimal kinetic mechanism wherein ethanol (or amine when amide is used as substrate) is released first from the enzyme surface, followed by the liberation of arginine. The enzymes are inactivated by phenylmethane sulfonyl fluoride which suggests the presence of an essential serine in the active sites of the enzymes. The enzymes may therefore be classified in the group of serine proteases.