A technique is described for using radiolabeled RNA complementary to human DNA as a probe for the specific identification of submicrogram concentrations of human DNA by formation of RNA-DNA hybrids. An example is given of its application to the semiquantitation of human DNA in human plasma, a substance that is ordinarily difficult to examine because materials are present that interfere with the usual colorimetric or fluorometric assays. An example is also given of the use of an analogous approach to analyzing rabbit serum for circulating bacterial DNA. Unique to the hybridization technique is a degree of specificity sufficient to identify specific base sequences and hence the origin of the DNA being detected, a point that may be important in the examination of circulating DNA reported to occur in patients with systemic lupus erythematosis. This technique may also be of value in clarifying the presently conflicting data regarding the occurrence of free DNA in the normal human circulation.