TheMURiNE Nk-1.1 antigen is found on murine natural killer cells.'1" 3'Since its definition by Glimcher et al.,'" anti-Nk-1.1 antiserum has been used by several groups of investigators to define a subset of NK cells, that have lytic activity to YAC or RL< Î 1 cells."" 3'Recently, we were successful in making monoclonal anti-Nk-1.1 antibody that has similar strain distribution as the conventional polyclonal antibody. This paper describes the screening process, and subsequent immunogenetic study of the monoclonal Nk-1.1 antibody. The mice we used for fusion were (C3H x BALB) F, immunized with spleen cells and bone marrow cells from CE mice. Spleen cells were first enriched for Nk-1+ cells by discontinuous BSA gradient as previously described. 13'After the fourth immunization, the mice were screened for antibody activity by Protein A rosette assay and by its ability to eliminate NK activity in a C dependent system.'3'Mice with good antibody titers were selected for fusion using Polyethyleneglycol 4000.'"'The myeloma cells were Sp2/0.'5'Macroscopic colonies of cells were detected in approximately 25% of the tissue culture wells and the supernatants were screened for cytotoxic activity against NK cells. C-dependent microcytotoxicity was adopted for use in the elimination of NK activity against YAC cells.'6'After two rounds of limiting dilution clone PK136 was selected to produce serum and ascites fluid in (C3H x BALB) F, mice. The titer by Protein A rosette assay was 1: 100-200; the titer by C elimination was 1: 100-1: 200. The IgG class produced by