H-2 alloimmunization evokes antibodies of differing cross-reactivity patterns among individual mice of one inbred strain. We investigated whether this is due to unknown variations at the time of bleeding or whether it remains constant for an individual mouse during long-term immunization. At various times during immunization, serum was collected from individual mice and it was tested on a panel of mouse and human lymphocytes. Sera from three donor-recipient combinations were examined. In some sera, a broadening of the cross-reactivity pattern occurred after prolonged immunization, but no major change in specificity was observed. We conclude that the differences in cross-reactivity pattern among sera from individual mice remain constant during the immunization period and do not reflect variations in the condition of the animals at the time of bleeding.