The anticancer agents 1-β-D-arabinofuranosylcytosine (ara-C) and methotrexate (MTX) were linked covalently to murine monoclonal IgG₁ antibody (H-1) with the use of dextran T-40. H-1 alone did not inhibit cell growth. The ara-C-(H-1) conjugate showed weaker cytotoxicity to an HLA-lacking cell line than ara-C, while ara-C-(H-1) and ara-C showed the same cytotoxicity to HLA-bearing cells. The MTX-(H-1) canjugste showed stronger cytotoxicity to HLA-bearing cells than MTX, while MTX-(H-1) and MTX showed the same cytotoxicity to cells lacking HLA. These results indicate that ara-C-(H-1) and MTX-(H-1) exert antibody-targeting cytotoxicity in vitro.