W e examined the ability of DNase I to digest DNA that was contained with DNA-anti-DNA immune complexes. IgG isolated from the sera of 20 patients with systemic lupus erythematosus (SLE) and containing antibodies to DNA was incubated with double-stranded DNA to form immune complexes. Excess DNase was added, and digestion of DNA was monitored by the conversion of DNA to TCA soluble products. IgG from 8 of the 20 SLE patients protected DNA from degradation by DNase in direct proportion to the amount of DNA bound to IgG as measured in the Farr binding assay. Using IgG from these sera, we showed that the DNA protected from degradation remained bound to IgG during digestion and was 35-45 base pairs in size. The size of this fragment is the same as that which has been proposed to be the minimal size necessary for monogamous bivalent binding of IgG to DNA. We therefore compared the ability of F(ab')2 and Fab' to protect DNA from DNase digestion and demonstrated that the bivalent F(ab')2 fragments were protective, but that the univalent Fab' fragments were not. These results suggest that some antibodies to DNA that bind to DNA via monogamous bivalent binding can protect a 35-45-base pair DNA fragment from DNase digestion. The implications of this finding are discussed with regard to the in vivo behavior and potential pathogenicity of small DNA-anti-DNA immune complexes.