Haemolymph from 6th instar larvae of Malacosoma americanum and 5th instar nymphs of Locusta migratoria was subjected to gel filtration on Sephadex G-200. Three protein fractions were produced in each case. Each fraction was analyzed for protein content and assayed for esterase and tyrosinase activity. The degree of resolution obtained by gel filtration was tested by Polyacrylamide "disc" electrophoresis. Each fraction displayed a specific type of enzyme activity and protein composition. The similarities between the haemolymph proteins of the two insects are discussed.