Agglutinins that reacted with red cells from members of a different species (heteroagglutinins) were detected in the serum of bovine fetuses beginning with about 5 % in the fifth month of gestation, with the frequency of reactive sera increasing to about 50 % near term. The sera were able to agglutinate red cells from an increasing number of species as fetal age increased, but only exceptionally equalled the number agglutinated by sera from the dams.Antibodies that reacted with bovine red cells (isoantibodies) were detected in bovine fetal sera (anti-L agglutinin, anti-K' hemolysin). However, no fetal serum was found that agglutinated the red cells of the respective dam.All fetal isoantibodies were inactivated by heat (56"C/30 min) and 2-mercaptoethanol (2-ME) treatment, whereas some heteroagglutinins in serum from one fetus were resistant to inactivation by these methods. This suggests that most of the fetal antibodies were other than IgG.