MHC class I inhibits NK cell recognition of pleural effusion-derived myeloma cell lines. Mean NK lysis percentage of KMS12BM and of KMS12PE are reported in panel A, while NK lysis of KMS21BM and KMS21PE are reported in panel B. BM and PE myeloma cell lysis are indicated with white and black columns, respectively. Antibody pretreatment of the PE targets are reported on abscisses: 1) complete medium, 2) anti-CD57 (TIB200) isotype control, 3) anti-MHC class I mAb A6 to 136 (IgM). The percentage of lysis reported was calculated with E/T ratio of 12:1. MHC class I masking restored myeloma susceptibility to NK recognition.