Bovine complement subcomponent C1q was purified, in a highly hemolytically active form, by a combination of precipitation with EGTA, ion-exchange chromatography, and gel filtration. Yield ranged from 22 to 28% as protein amounts, and the activity of final preparations was in the range of 2 X 10(13)-4 X 10(13) effective molecules/mg. The molecular weight of undissociated C1q was 407,000, as determined by polyacrylamide gel electrophoresis containing sodium dodecyl sulfate (SDS). C1q was shown to be composed of two non-covalently linked subunits of approximately 46,000 and 45,000 molecular weights in a molar ratio of 2 : 1. On reduction, the higher molecular weight subunit gave two chains having approximate molecular weights of 23,600 and 22,200 in equimolar ratio, and the lower molecular weight subunit gave one chain with a molecular weight of approximately 22,000. C1q contained hydroxyproline, hydroxylysine, a high percentage of glycine and approximately 9% carbohydrate and 14.8% nitrogen. The absorption coefficient (A 1% 1cm) in 300 mM NaCl was found to be 7.3 +/- 0.12 at 280 nm. From these results, overall molecular structure of bovine C1q looks similar to that of human complement subcomponent C1q.