Three hybridomas, designated C2, V17, and B2, were produced from BALB/c mice after immunization with measles virus. All three were directed against the virus hemagglutinin (HA). The HA is a structural peptide of the virus and constitutes a major target for the host immune response during measles infection. The monoclonal anti-HA antibodies have biological functions such as (i) measles virus neutralization in vitro, (ii) binding to acutely and persistently infected cells, and (iii) inhibition of HA-mediated Rhesus monkey erythrocyte agglutination. Different idiotypes, designated HAMM-1, HAMM-2, and HAMM-3, were defined on C2, V17, and B2, respectively, by syngeneic anti-idiotype sera against those three monoclonal antibodies. A limited cross-reactivity with the HAMM-1 idiotype was detected in sera from some BALB/c mice immunized with measles virus. The anti-idiotype sera could significantly inhibit the biological functions of the HAMM-1 and HAMM-3 idiotypes bearing monoclonal anti-HA-antibodies. This suggests a possible role for auto-anti-idiotypes in the immune response after infection with measles virus.