[en] The unavailability of serological tests for detection of several not yet characterized infectious agents transmitted by blood transfusion or by blood products prompted the development of alternative tests based on utilization of labeled nucleic acid probes specific for genomes of each of these agents. The prerequisite for the preparation of such probes is the demonstration in human plasma of nucleic acid sequences distinct from those present in host DNA or in genes of already characterized viruses occurring in plasma of infected individuals. To accomplish this, ultrasensitive tests for nucleic acids not dependent on their base sequence are needed. The authors describe a radioimmunoassay (RIA) for picogram quantities of DNA. Plasma (serum) specimens are treated with proteinase K in the presence of sodium dodecyl sulfate and extracted with phenol. Nucleic acids are precipitated with ethanol in the presence of dextran (mol.wt. approx. 5X10⁵) as carrier. Subsequently, DNA from the redissolved samples is adsorbed onto polylysine-coated wells of microtiter plates and detected by a double-antibody RIA using anti-DNA autoantibodies from NZB/NZW mice and ¹²⁵I-labelled antibodies to mouse immunoglobulins. DNA which did not hybridize with human DNA was detected by this method in sera containing hepatitis B virus used as a model system. (Auth.)