The analysis of the interaction of ethanolamine-O-sulphate with 4-aminobutyrate transaminase revealed that the inhibitory effect is exerted upon the substrate subsite of the active site of the enzyme in aldimine form. The inhibition in irreversible. The inactivation rate versus pH-curve was shown to have a sigmoid character with inclination point at neutral pH. The study of inhibition kinetics by the Kitz and Wilson method revealed a complex inhibitory pattern compatible with a minimal two-step mechanism. Rate constant of inactivation was found to be equal to 0.22 min-1 and the value of the inhibitory constant--to 1.1-10(-2) M.