A murine IgM monoclonal antibody specific for human Ia antigen was purified by Sephacryl S-300 chromatography from ascites obtained from BALB/c mice inoculated with the antibody-secreting hybridoma. The anti-Ia antibody (H-2) was reacted with Neocarzinostatin (NCS) in the presence of carbodiimide. The resulting mixture was chromatographed on a Sephacryl S-300 column. The first fraction was shown to contain H-2-NCS conjugate, but not free NCS, by the elution profile, as well as by the Sarcina lutea growth inhibition assay. A membrane immunofluorescence test with anti-NCS and anti-mouse immunogloblin demonstrated specific localization of H-2-NCS on the cell surface of Ia-bearing NALL-1 cells. No localization of NCS was shown when the cells were incubated with free NCS. H-2-NCS inhibited the growth and (3)H-TdR incorporation of NALL-1 cells 100 times more potently than free NCS alone, when the cells were reacted with the drugs at 4℃ for 30 min. and culutured further for 14 hrs. These results indicate that the H-2-NCS cnojugate retained both NCS and antibody activities, and exerted antibody-targeting NCS cytotoxicity in vitro. Thus the conjugate should be useful for cancer immunochemotherapy.