Available lymphocytotoxic antisera permitted the clear partition of the Bw21 antigen into two distinct components, Bw21.1 and Bw21.2. Bw21.1 is associated with W4 and is approximately twofold more frequent than the W6‐associated Bw21.2. Cells of either Bw21 subtype were capable of absorbing specific anti‐Bw21.1, anti‐Bw21,2, and anti‐Bw21 (21.1 + 21.2) antibodies. Further, an F(ab')2fragment prepared from an anti‐Bw21.1 serum blocked cytotoxicity of anti‐Bw21.1, anti‐Bw21.2 and anti‐Bw21 (21.1+21.2) sera. Based on the crossreactivity (by absorption) and blocking data, a model is proposed relating the Bw21 subtypes and the W4