We propose a novel and simple assay for the real-time differentiation between carbamate and organophosphate inhibition of cholinesterase, based on our observations of the kinetic behavior of inhibited enzyme. The assay of carbamylated cholinesterase activity over time follows a non-linear kinetic pattern, whereas that of phosphorylated enzyme activity is linear. This feature can be exploited to differentiate between carbamate and organophosphate cholinesterase inhibition. The non-linear pattern characteristic of carbamates is easily discernible at degrees of inhibition of 40% or more. In this setting, cholinesterase activity ought to be measured continuously for about 1 h to obtain the kinetic pattern of enzyme activity. The initial activity, measured during the first 5 min of assay, represents the activity of enzyme in vivo. In vitro reactivation of inhibited cholinesterase allows the estimation of full potential activity of enzyme prior to poisoning, so that percentage of inhibition can be calculated. Reactivation of carbamylated cholinesterase is obtained by the incubation of diluted enzyme at 37 degrees C for 2.5 h prior to assay, whereas phosphorylated (non-aged) enzyme is reactivated by a 30 min incubation with oximes. In cases of mild exposure to cholinesterase inhibitors (< 40% inhibition), the response of enzyme to in vitro reactivation serves as a complementary test for exposure and for the nature of the inhibitor. All the results presented in this work refer to plasma cholinesterase. Erythrocyte cholinesterase was found to behave very similarly to plasma enzyme and its results have not been reported here.