Described herein is the use of polycationic solid supports in the purification of nucleic acids from solutions containing contaminants. The nucleic acids non-covalently bind to the support without significant binding of contaminants permitting their separation from the contaminants. The bound nucleic acids can be recovered from the support. Also described is the use of the supports as a means to separate polynucleotides and hybrids thereof with a nucleotide probe from unhybridized probe. Assays for target nucleotide sequences are described which employ this separation procedure.