A microassay for the determination of iodide and iodine-containing compounds based on the Sandell-Kolthoff reaction was developed by O’Kennedy et al., (1989). Non-radioactive iodine was then used to label antibodies for immunoassay. The sensitivity of this immunoassay has been shown to be comparable to that obtained in an ELISA when the enzyme used was horseradish peroxidase. This study was carried out to investigate the feasibility of labelling DNA probes with non-radioactive iodine. Nucleic acids have previously been labelled with 125Iodine and these iodination procedures were adopted. Labelling of nucleic acids was carried out both chemically using the oxidizer Iodo gen, and enzymatically via the incorporation of 5-iodo-2’-deoxycytidine 5 ’-triphosphate by random priming. 2% and 38% incorporation was observed for these methods, respectively. The degree of iodination achieved was determined using the iodide microassay. HPLC of digested nucleic acids was also carried out to measure the incorporation of iodine. The iodide microasssay was modified and then validated in the range 1- 10ng/ml potassium iodide. The catalytic effect of 5-iodocytidine, and related compounds, in the iodide microassay was also considered. The iodide assay was performed on nitrocellulose paper to investigate the feasibility of applying this assay in a dot blot format. The limit of quantification for iodide in this format was 1 .0pg/ml. To use cold-iodine as a direct label for DNA probes a more sensitive iodide microassay would be required. For this reason a number of chemiluminescent systems, which were quenched by iodide, were investigated. However, the lowest limit of quantification obtained for the microassays considered was 10ng/ml iodide. The use of iodine as an indirect label for DNA probes was also examined and polyclonal antibodies to 5-iodocytidine were produced.