A simple method for the isolation and characterization of DNA-DNA and DNA-RNA hybrid molecules formed in solution was developed. It was based on the fact that, in appropriate salt concentration, such as 5% Na2HPO4, DNA in either double-stranded (DNA-DNA or DNA-RNA) or single-stranded forms, but not free nucleotides, can bind to diethylaminoethylcellulose disc filters (DE81). Thus tested samples were treated with the single-strand-specific nuclease S1 and then applied to DE81 filters. The free nucleotides, resulting from degrading the single-stranded molecules, were removed by intensive washing with 5% Na2HPO4, leaving only the hybrid molecules on the filters. The usefulness of this method was illustrated in dissociation and reassociation studies of viral (SV40) or cellular (NIH/3T3) DNAs and DNA-RNA hybrid molecules. Using this technique the reassociation of denatured SV40 DNA was found to be a very rapid process. Dissociation studies revealed that the melting curves of tested DNAs were dependent on salt concentration. Thus the melting temperatures (tm) obtained for SV40 DNA were 76 degrees C at 1 X SSC (0.15 M NaCl-0.015 M sodium citrate) and 65 degrees C at 0.1 X SSC, and for NIH/3T3 DNA 82 degrees C at 1 X SSC and 68 degrees C at 0.1 X SSC. MuLV DNA-RNA hybrid molecules were formed by annealing in vitro synthesized MuLV DNA with 70S MuLV RNA at 68 degrees C. The melting temperature of this hybrid in the annealing solution was 87 degrees C. Another important feature of this procedure was that, after being selectively bound to the filters, the hybrid molecules could efficiently be recovered by heating the filters for 5 min at 60 degrees C in 1.5-1.7 M KCl. The recovered molecules were intact hybrids as they were found to be completely resistant to S1 nuclease.