The paper describes a cattle serum antigen (McB1) which is controlled by a dominant gene (McB1), independent from those controlling the two macroglobulin markers McA1 and McA2 as well as from that controlling the low-density lipoprotein marker Ld1A1. McB1 is located on a high-molecular-weight serum protein which, very likely, is an alpha2-macroglobulin.