Hybridization of murine myeloma cells P3-X63-Ag8.653 with splenocytes from a BALB/c mouse immunized with the syngeneic anti HLA-DR1,4,w6,w8,w9 MAb AC1.59 resulted in the development of 108 hybridomas secreting antiidiotypic antibodies. 100 of them inhibited the binding of MAb AC1.59 to target cells. Detailed analysis of the antiidiotypic MAb F5-444, F5-830, F5-963, F5-1126, F5-1336, and F5-1419 showed that all of them recognize idiotopes within or spatially close to the antigen combining site of MAb AC1.59. In cross-blocking experiments, the six antiidiotypic MAbs cross-blocked each other. It is likely that the six MAbs recognize spatially close, but not identical idiotopes because they elicited antiantiidiotypic antibodies of different or similar, but not identical specificity and differ in their ability to elicit anti-HLA class II antibodies. The latter, which were found only in sera from BALB/c mice immunized with antiidiotypic MAb F5-444 and F5-830, mimic the specificity of MAb AC1.59 and express the idiotope defined by the immunizing antiidiotypic MAb. These results indicate that the MAb F5-444 and F5-830 are antiidiotypes beta and the remaining four are antiidiotypes gamma.