Caries in exposed root surfaces is becoming a significant problem in dentistry. Varnish applications have interesting preventive possibilities against root caries. The effect of such a varnish, containing two antimicrobials, was studied on the demineralisation of roots with a twofold aim. Firstly, the efficacy of the varnish was investigated in situ as a function of the demineralisation period. Ten participants wore sound intact roots treated with the varnish in an intra oral appliance for 2, 4 and 6 weeks. Secondly, the effect of the varnish on root demineralisation after removal of the varnish was measured. The results showed that the reduction, in demineralization of about 80% after 2 weeks, decreased in magnitude with the length of the in situ demineralisation period. After 4 weeks there was no longer statistically significant difference compared with untreated roots. The acid attack by plaque was, however, substantial in this model; after 6 weeks in situ untreated roots were so strongly demineralised that microradiography was no longer possible anymore. In the second experiment in which the varnish was removed 2 days after application there was a trend toward a reduction of about 30% in root demineralisation after 2 weeks. This was not statistically significant when compared with no application.