This study was carried out to investigate the retention of several fissure sealant materials in occlusal fissures in vitro. 400 sound human teeth were coated with varnish leaving occlusal and smooth surface sites unpainted and exposed for experiment. The effect of a number of acid-‘conditioning’ fluids on the enamel surface was also investigated. Window regions were exposed to one of the following solutions: phosphoric acid in the concentration range 20–70%, 50% phosphoric acid buffered with 7% zinc oxide by weight, 5 and 50% citric acid, 10% polyacrylic acid and 5% neutral and acid solutions of EDTA. Exposure times varied from 1 to 5 min. Sealants employed were Epoxylite 9070 and 9075, Elmex protector, Nuva-Seal and a material under development, TP 226. The first three materials are chemically polymerized whereas the latter two are activated by UV light. A UV fluorescent dye was added to some of the materials. Sealants were applied to previously conditioned enamel surfaces. Whole teeth and teeth slit into half were viewed with incident light. Ground sections from half of the specimens were then examined by transmitted, polarized and UV light fluorescence microscopy, and with the scanning electron microscope. Sealed teeth were also exposed to buffered lactic acid for periods ranging from 1 to 5 days. One half of the number of ground sections from all experiments was then demineralized in 10% HCl. The fitting surfaces of the sealant remaining were examined by light and electron microscopy.