Previous in vivo experiments demonstrating the existence of humoral immunity in cockroaches prompted us to investigate the composition and number of proteins that are newly induced or increased in quantity in the haemolymph of immune cockroaches compard to saline-injected control animals. SDS-PAGE analysis showed that six different haemolymph proteins of molecular weights 220,000, 162,000, 115,000, 102,000, 95,000 and 45,000 were induced in response to immunization by honeybee toxoid. However, only the 115,000 and 102,000 MW proteins consistently increased in quantity in the haemolymph of females immunized with various other soluble protein antigens, such as phospholipase A2 toxoid and inactivated creatine phosphokinase. Other experiments showed that the 115,000 MW band was not induced in immunized male cockroaches. Therefore, only the 102,000 MW protein was consistently enhanced in all samples of immune haemolymph from animals injected with various antigens. In addition, this protein was found in precipitates recovered from assays where immune haemolymph proteins were precipitated by the antigen. Gel filtration chromatography showed that this induced haemolymph protein is large, and bears an apparent native molecular weight of 600,000 ± 80,000. This suggests that the 600,000 MW protein may consist of multiple subunits of 102,000.