The immunochemical reactivities of the alpha A and alpha B subunits from bovine alpha-crystallin have been compared using 5 monoclonal antibodies and 2 polyvalent antisera. Each subunit bound the same maximum amount of antibody, regardless of its source, and each subunit was able to completely displace alpha-crystallin from its antibodies. One monoclonal antibody (463-12.2) and mouse anti-alpha B polyclonal antiserum bound equally well to the two subunits; with the other monoclonal antibodies and an anti alpha-crystallin antiserum, the affinities for the alpha A chains were about 10(3) fold higher than those for the alpha beta chains. These observations indicate that the alpha A and alpha B subunits of bovine alpha-crystallin share several similar, but not necessarily identical, cross-reacting antigenic determinants. The reasons for the differences between these observations and those of other investigators are discussed.