Holmes1) first demonstrated that tomato and tobacco stems inoculated with a severe strain of tobacco mosaic virus (TMV) and incubated at 35C became contained mild strains. Lebeurier and Hirth2) studied on serial passage of a common strain of TMV in large leaf disks of tobacco at successively higher temperatures, and isolated a thermophilic strain which developed actively at 36C but not at 24C. The plant of Nicotiana glutinosa responds always with necrotic local lesions to TMV infection at

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... atures below about 30C. At higher temperatures, however, the type of reaction changes, the local lesions become chlorotic instead of necrotic, and the plant becomes systemically infected and develops mosaic symptoms3). This paper deals with a new procedure for induction of attenuated strains of TMV by means of serial passages through the intact plants of N. glutinosa alternately infected with the virus systemically at higher temperatures of 30, 35, 37 and 38C and locally at usual temperature of 25C. The original virus inoculum used was obtained from the plant of N. tabacum cv. Samsun systemically infected with an ordinary strain of TMV, which had been maintained for long time in the greenhouse and was passed twice through the necrotic local lesions of N. glutinosa leaves at 25C just before inoculation of the donor tobacco plants. Before initiating the serial passage, three clear necrotic local lesions, developed on the leaves at 25C of N. glutinosa inoculated with the original virus inoculum diluted 105-fold with distilled water, were named provisionally as SA, SB and SC, and were separated individually and macerated with the small glass rod on the slide glass adding a small quantity of distilled water. First, each of three seedlings of N. glutinosa was inoculated with each virus inoculum derived from the necrotic local lesions SA, SB and SC. They were kept at 30C for several days and became systemically infected.