The knowledge about permeate flux decline regarded to apitoxin can be a useful tool to allow industrial fractionation of the mixture. Hermia's models were used to predict the fouling mechanisms during cross-flow ultrafiltration of bee venom in 10 kDa regenerated cellulose membrane. Three different apitoxin concentrations (0.2 to 1 g/L) were tested at 1 bar and 0.08 m/s. Fouling mechanisms varied as a function of feed content. Membrane cleaning with sodium dodecyl sulphate was effective. Phospholipase A(2) rejection coefficients were a function of membrane fouling, varying from 35 to 93%.