The influence was studied of the size of DNA-anti-DNA complexes on their capacity to induce phagocytosis and degradation by monocytes and neutrophils from human blood. An estimate of the size of the complexes was obtained by precipitation analysis, sucrose-gradient velocity sedimentation and filtration over Nucleopore filters. The results show that only those complexes are processes that precipitate in 15 min at 3000 x g. These complexes have an S value of more than 25000 and a diameter of more than 0.4 micrometer. Likewise, tetanus-anti-tetanus complexes also had to be precipitable in 15 min at 3000 x g to be processed by the phagocytes.