Methods were developed for titrating bovine C3b-inactivator, C2, C3 and C4 by non-hemolytic means, and for assaying by hemolysis all the components of the bovine classical complement system except C2. All components were detected at serum dilutions above 1:1000, and some at dilutions above 1:100,000. C1, C4, C5, C7 and C9 titers were very high in adult bovine serum, and C2 and C8 were relatively low. C1, C2, and C8 were quite heat-labile at 56 degrees C, and C7 was moderately so, while C3 amd C6 titers increased after heating due to inactivation of a heat-labile inhibitor. Fetal bovine serum contained approximately 1-3% of adult levels of conglutinin, C1 and C6, and 5-50% of adult levels of the remaining components except C3. C3 antigen was found, but C3 functional activity was undetectable in most fetal bovine sera, though present at low levels in a few.