In order to meet the present increased demands, we have tried to improve the methods to produce human monoclonal antibodies by using a SCID mouse grafted with human mononuclear cells. Initially, we gave an anti-asialo GM1 antibody to a SCID mouse to suppress the NK activity, as a pretreatment. Then, fifty million human mononuclear cells (MNC) from a healthy volunteer were injected intraperitoneally to the SCID mouse so as to construct a human immune system in the mouse (PBL-SCID mouse). We immunized the mouse with a synthetic peptide (pep 190) conjugated with KLH four times. The spleen cells taken from the immunized PBL-SCID mouse were fused with (mouse x human) heteromyeloma cells. The hybridoma cells were selected in GIT medium containing HAT and IL-6. Among 68 hybridoma-growing wells, we obtained one hybridoma clone (#41-1-4) which secreted a specific antibody to pep 190. The reactivity of this monoclonal antibody was tested by ELISA and the specificity of this antibody was confirmed by an absorption test with different kinds of proteins. This paper is the first report of the successful production of peptide-specific human monoclonal antibody by active in vivo immunization using a PBL-SCID mouse. By this active in vivo immunization system using a PBL-SCID mouse, human monoclonal antibodies for any sort of peptide antigens may easily be made available.