ABSTRCT The effect of different levels of mercury substitution an the rate and extent of hybridisation of globin mRNA with a complementary DNA (cDNA) copy has been investigated. It was found that mercuration significantly reduces both the rate of hybridisation and the extent of the reaction, but that these effects are abolished when at least stoichiometric amaunts of 2merma#oethanol are included in the hybridisation medium. As a preliminary to using this technique to isolate specific groups of sequences after longterm hybridisations, we have investigated both the rate of demercuration of RNA and its retention on thiol-sepharose columns after extended incubation under cammonly employed hybridisation conditions at 430 or 600. Retention was essentially quantitative even after incubation times of 300 hours at 430, but decreased significantly after 48 hours at 600. It is concluded that thiol.eepharose chromatography offers considerable advantages over hydroxyapatite chromatography for the recovery of hybridised sequences, particularly with regard to the lower levels of non-specific binding obtained and its ability to distinguish directly between DNA-DNA and DNAHg RNA hybrids.