Thallium and its compounds have a high toxic potency. For the production of particularly resistant sorts of cement, additives containing thallium, among other things, are employed. In conformity with our present toxicological knowledge, the determination of the concentration of thallium in the urine must be considered a suitable parameter for the assessment of the presence of thallium in the body. Occupational-medical preventive examinations were carried out in a total of 128 male employees from all areas of production in three cement factories. An exposure of these employees to thallium was objectified by analyses of the roasted pyrites employed and the dust from the electric filter. The study included the questioning of the employees with respect to their previous history of health, and also a physical examination aimed at detecting clinical symptoms of a possible effect of thallium. The analysis of thallium was carried out in samples of "spontaneous" urine. The analytical method employed was flameless atomic absorption spectroscopy (carbon-rod atomizer). In part, the group of persons investigated revealed excretions of thallium slightly or moderately above the normal level (range: < 0.3-6.3 micrograms/g creatinine). As the upper normal limit of thallium excretion, we computed a value of 1.1 micrograms/g creatinine. In no case, however, did the case history data or the findings of the physical examination reveal any indication of the symptoms characteristic of thallium poisoning.