In order to select individuals of Rhododendron mucronatum plants that have a high capacity to decontaminate nitrogen dioxide (NO 2 ), which is a major air pollutant, a number of lines prepared by cutting propagation were analyzed for their uptake of NO 2 . In 1996, 1452 different lines (referred to as the "96-plants") were fumigated with 4 µl/l 15 NO 2 for 8 hours, and the total nitrogen content of the leaves, derived from NO 2 , was determined and reflects the plant's NO 2 uptake capacity. A 56-fold difference was observed between the individuals with the highest and lowest capacities. About 100 lines each from the high-, middle-and low-capacity groups were propagated and assayed for NO 2 uptake in the following year (the "97-plants"). The maximum variation in NO 2 uptake was 5.6 times. In 1998, 227 lines propagated from the 96-plants and 95 lines from the 97-plants were assayed for NO 2 uptake (the "98-plants"). The variations in NO 2 uptake in 97-and 98-plants were a maximum of 4.9 and 5.2 times, respectively. The reason (s) for the disappearance of the initial large variation in NO 2 uptake is (are) not yet known. Those lines showing high and low NO 2 uptake capacities will be useful when studying the genes involved in this important roadside tree.