The inactivation of soybean lipoxygenase-1 and of rabbit reticulocyte lipoxygenase by five selected acetylenic fatty acids was studied. In all cases the inactivation was time-consuming and depended on the concentration of the inactivator. The inactivation kinetics was measured and the data were fitted to a kinetic model based on the assumption of catalytic self-inactivation. The kinetic constants (Km-value and inactivation rate k2) calculated indicated that 7,10,13-eicosatrienoic acid was the most powerful inactivator for the soybean enzyme followed by 8,11,14-eicosatrienoic acid. The occurrence of an additional triple bond between C-4 and C-5 or between C-5 and C-6 strongly reduced the suicidal rate. With the reticulocyte enzyme, only small differences in the reactivities towards various acetylenic fatty acids have been observed.