When EAC43b were treated with heated serum in EDTA, reactivity with bovine conglutinin appeared rapidly, even at 0 degrees C, and almost simultaneously with the loss of C3b rosetting capacity. At the time conglutinability first appeared, there was no detectable decrease in I-A or hemolytic C3 activity, and no detectable C3 antigen release from the cells. With prolonged exposure to heated serum in EDTA, I-A (immune adherence) and hemolytic C3 activity were lost. If this exposure was at 37 degrees C, C3 antigen became strongly detectable in the supernatant fluid, and eventually conglutinability was markedly reduced or lost, whereas C3d rosettes were unaffected. We suggest that bovine conglutinin reacts with some early product of C3b degradation, rather than with C3d, and propose that this intermediate be designated C3k. We have developed a semi-quantitative assay for bovine conglutinin, utilizing a Coulter Counter to register the decrease in total particles due to red cell aggregation. By using this method, we have detected conglutination with mouse complement (C) as well as with that from man and the guinea pig.