The present work is concerned with a sensitive and fast micromethod for separation of single- and double-stranded molecules of nucleic acid by hydroxyapatite (HAP) thin-layer chromatography. The thin layers were obtained by precipitation of ground HAP particules into the surface of the plates in water. Chromatography in sodium phosphate buffer makes it possible to separate from 1 to 50 micrograms of nucleic acids for 30--50 sec. Thereby double-stranded molecules remain at the starting line, whereas single-stranded DNA or RNA follow up the solvent. For quantitative assay of nucleic acids by HAP thin-layer chromatography, the plates were scanned in UV light, radioactivity was measured without extracting substances from HAP and DNA and RNA were eluted with the help of phosphate buffer. A simple and accurate determination method has been suggested consisting in dissolving HAP in perchloric acid followed by hydrolysis of nucleic acids and spectrophotometry of solutions. The retrieval of the material after chromatography in 99 +/- 2%, the mean determinations error is 2--3%. The conditions are described for extraction, after thin-layer chromatography, of desalted and concentrated DNA, ready for use in later experiments. The paper describes a method: for determination of the degree of DNA nativity; quantitative determination of DNA in solutions, containing admixtures; separation of synthesized RNA from its precursors and from the DNA template; assay of DNA thermostability; investigation of the kinetics of DNA reassociation and DNA-DNA hybridization. Some results obtained from hydroxyapatite thin-layer chromatography are discussed.