The cross-resistance is one of the most important problems in the chemotherapy of infectious diseases and some important suggestions for chemotherapy have been given by cross-resistance studies. A new antibiotic kanamycin sound by Umezawa, et al ., will be usually utilized for tuberculous patients who have been already treated by several courses of chemotherapy with other drugs, especially with two or three drugs of streptomycin, isoniazid and PAS, and, in some cases, also of viomycin, cycloserine, and 4-acetylaminobenzaldehyde-thiosemicarbazone. Subsequently, the problem of cross-resistance and of selection of chemotherapeutic drugs that may be used in combination with kanamycin must be very important for such patients.

Cross-resistance studies have been usually made by utilizing not any constant size of inoculum and their meanings have not been accurately interpreted. According to our opinions, however, it appears that cross-resistance may occur from two origins; the one is a decreased susceptibility to a drug of “individual mutants” resistant to an another drug and the other is an increased mutation rate to a drug resistance in a “mutant population” resistant to an another drug. Kanamycin susceptibility of various drug-resistant mutants have been studied in the foregoing paper¹⁾ and, following the study, mutation frequency to kanamycin resistance in various drug-resistant strains will be studied in the present paper.