peripheral blood from a patient with chronic myelomonocytic leukemia was centrifuged over ficoll paque to separate the mononuclear cells after washing and counting the cells were plated on agar as a concentration of 10 5 per plate after incubation in medium containing monoclonal antibodies bion 1 or 1c1 with or without il 3 for 14 days at 37 u00b0 c the number of arising colonies were counted by mycroscopical examination each cell cluster containing more than 40 cells was counted as a colony previous experiments have shown that the putative f u2032 g u2032 loop of u03b2 c contains a common binding site for il 5 gm csf and il 3 woodcock et al 1996 wo 97 28190 we have now produced a blocking compound represented by moab bion 1 by immunizing mice with cos cells transfected with a cdna coding for domain 4 of u03b2 c screening of hybridoma supernatants was performed on a cho cell line expressing domain 4 of u03b2 c one hybridoma cell line was identified which produced a moab which specifically recognized this cell line and not a parental cho cell line not expressing domain 4 of u03b2 c this moab was termed bion 1 and was characterized in biochemical binding and biological experiments bion 1 recognizes domain 4 as well as wild type u03b2 c moab bion 1 was tested for reactivity against cell lines transfected with u03b2 c and against primary cells known to express il 5 gm.