The epitopes recognized by six new BALB/c hybridomas specific for the protein antigen hen egg-white lysozyme (HEL) were mapped in detail. Although fine specificities of all the antibodies were distinct, many of the epitopes overlap in complex patterns. The antibodies could be grouped into three complementation groups, one of which also included the previously characterized HyHEL -5 which is specific for Arg68 . Complex interactions were observed among the antibodies, both among and within complementation groups, including nonreciprocal competition and enhanced binding. Two of the complementation groups mapped near the catalytic site in a new antigenic region. The antibodies HyHEL -8 and HyHEL -10 had very similar and over-lapping specificities, and may recognize very closely related epitopes. The results suggest that the epitopes may form a continuous antigenic surface, and that antigenic regions correspond to structural domains defined by the tertiary structure of HEL.