The hybridoma cell line KM50 originally produces a monoclonal antibody at a concentration of approximately 40 mg ml(-1) in ascites. To investigate the possibility to apply this expression system to the production of useful proteins, the cDNA encoding human granulocyte colony-stimulating factor was inserted by homologous recombination into just downstream of the promoter of the active immunoglobulin heavy chain gene of KM50. Site directed integration of targeting DNAs resulted in the disruption of expression of the immunoglobulin heavy chain proteins with a frequency of 1 in 10 approximately 100 G418-resistance transfectants. One of the monoclonal antibody-deficient transfectants produced25 ng ml(-1) of granulocyte colony-stimulating factor in the supernatant of its cell culture the number of molecules of which corresponds to that of the monoclonal antibody originally produced by KM50. However, when this transfectant was injected intraperitoneally, it produced only a 9 mug ml(-1) concentration of granulocyte colony-stimulating factor in ascites, which is approximately 3 orders of magnitude less than the monoclonal antibody. This method may be applicable to production of other recombinant proteins, although further optimization in the conditions of production would be needed in order to reach much higher yields.