Measurements of urban indoor fine particle <1 µm (PM₁) mass concentrations were carried out in a lecture room, restaurant and two types of office. Twenty-four-hour concentrations of PM₁ were sampled from April to July 2000 by low-volume Harvard impactors. The 24-hour concentrations were in the range from 7.69 to 214.62 µg·m^–3. The lowest average concentration was found in a non-smoking office (11.77 µg·m^–3, range 7.69–17.27 µg·m^–3), the highest concentration in a restaurant (169.09 µg·m^–3, range 131.91–214.62 µg·m^–3). In the lecturing room (average 17.98 µg·m^–3, range 11.91– 24.59 µg·m^–3), the particle concentrations corresponded to the number of people present during the day. In the offices, noticeable differences were found between working days and weekends or days when staff were not present. A strong influence of smoking on the concentrations of small particles was confirmed for the restaurant.