The inhibitory effects of a naturally occurring diterpenoid furanolactone, columbin, on partially purified acidic phospholipase A2 (PLA2) from Naja nigricolis was investigated. Columbin inhibited the N. nigricolis PLA2 in a dose related pattern with an IC50 value of 2.5 microM. Double reciprocal plots of initial velocity data of inhibition by columbin revealed a non-competitive pattern. The KM remained constant at 19 microM, while the Vmax changed from 54 micromoles/min/mg to 32 micromoles/min/mg and 20 micromoles/min/mg in the presence of 2 and 10 microM of columbin, respectively. Extrapolated Ki values were 3 and 6.28 microM at 2 and 10 microM inhibitor, respectively. Columbin also inhibited PLA2 hydrolysis of ghost RBC in a dose-dependent fashion. At least 70% suppression of PLA2-catalysed haemolysis of RBC was observed in the presence of 2 microM columbin.