GM-CSF is a glycoprotein that activates growth and differentiation of hemopoietic progenitor cells, usefull to reverse or prevent chemotherapy secondary leukopenias. In spite of the differences in the chemical structure, due to the variable glycosydic content of the forms produced in bacteria, yeast and mammalian cells, all of them have biological activity. Independently, antigenicity of non glycosylated recombinant human proteins may have relevance in the choice of the host system for the production of factors for clinical use.To study the undesirable immune response in the results of the therapy, we used the following strategy: i) To obtain the glycosylated hormone, CHO dhfr⁻ cells were manipulated by genetic methods to produce human GM-CSF under the control of the adenovirus major late promoter. ii) A panel of GM-CSF MAbs was produced from hybridoma cells obtained in our lab. We analyzed the specificity of the MAbs taking into account their neutralizing capacity. Some of them neutralized the in vitro biological activity of the hormone, with different affinities for GM-CSF from several sources. Patients under treatment with the commercial non-glycosylated GM-CSF develop antibodies and undesirable responses. Competition experiments between the neutralizing MAbs and these human antibodies would demonstrate the importance of the immune response in the choice of the host system for its production.