Biological effects of fluorine on man and other living organisms are briefly described. Furthermore, results of a study on fluorides content in drinking water sampled in the region of Gdańsk and some neighbouring towns as well as in some tooth-cleaning materials (tooth-pastes, tooth-powder, gel), are presented. Fluorides were assayed by spectrophotometry using microdiffusion procedure with the mixture of concentrated perchloric acid and sliver sulfate to release fluoride from the sample. Fluoride concentration in drinking water sampled in Malbork was found to range from 1.9 do 3.0 mg F- /L. In the water sampled from Gdańsk-Stogi region, periodically increased fluoride levels were also found (from 1.8 to 2.3 mg F- /L). Lower concentration of fluorides (from 0.12 to 1.35 mg/L) was noted in water samples taken from the other places in Northern Poland. Foreign and home tooth pastes were classified into three groups according to fluorides concentration determined: low level ranging from 0.0 to 10 mg/J00 g, middle level ranging from 26.0 to 60 mg/100 g, and high level ranging from 100 to 209.0 mg F- /100 g. Dental; preparation „Elmex Gelle" was found to contain the highest level of fluorides (1250 mg/100 g). Caries prevention by the supplementation of drinking water with fluorine or by the application of tooth pastes with this agent should be realized with great accuracy. The addition of fluorides to local