We have studied the effect of the number of days following the last antigen boost on the specificity of monoclonal antibodies produced by B cell hybridomas using spleen cells of mice immunized with human red cells of the A blood group. We showed, as previously observed by others, that the highest numbers of monoclonal anti-human red blood cells were obtained in fusions done 3 and 4 days after the final boost. However differential screening of the hybridoma cultures showed that the majority of the monoclonal antibodies reacting with the A blood group antigen were obtained in fusions done only 2 days after the last antigen injection. These results show that the delay between the final boost and the fusion experiment can influence not only the total number of antibody-secreting hybridomas but also the specificity of the antibodies produced.