The results presented here indicate that the sheep and chicken cell hemagglutinins of normal C57BL/6Sn sera exhibited significant fluctuations over a period of 6 months. It was found that the mean titer levels of sheep and chicken cell hemagglutinins have, in general, a parallel distribution over a corresponding period of time. A sex difference in the hemagglutinin mean titer levels for both sheep and chicken cells has been demonstrated, i.e., females have significantly higher hemagglutinin titers than do males.