The uptake in vitro was studied of 3H-labeled DNA-anti-DNA complexes by neutrophils and monocytes from human blood. Complexes were prepared from 3H-labeled circular double-stranded (dS) DNA of bacteriophage PM2 and anti-dsDNA-containing sera from patients with systemic lupus erythematosus. After phagocytosis, cells and medium were separated. The cells were treated with DNase to remove adherent and noningested complexes before the cell-associated radioactivity was counted. Thus, only complexes inside the cells were measured. The medium was analyzed for acid-precipitable radioactivity. In this way, we found that neutrophils only phagocytose the complexes, whereas monocytes phagocytose the complexes and degrade the antigen. In contrast, both types of phagocyte degraded the antigen in tetanus-anti-tetanus complexes. The degradation took place after phagocytosis, inside the cells. The difference in DNA degradation between neutrophils and monocytes correlated with the difference in acid DNase activity of the lysosomal fractions: monocytes contained DNase activity, neutrophils did not. With complexes made from DNA with 131I-labeled anti-DNA, we found that both cell types degraded the antibody. Uptake of complexes and degradation of antigen increased with incubation time and cell concentration and was saturable with respect to complex concentration. The processes were inhibited by 5 mM mono-iodoacetic acid or by low temperatures.