An analysis of 246 patients with positive smear present bacilli and negative cultures revealed that in only 9% of the cases no organized previous treatment had been applied, although almost half of these had benefited from short courses with streptomycin associated with penicillin before an exact diagnosis of tuberculosis had been made. Since the remaining 91% had had 1--2 months of chemotherapy, which frequently included rifampicin, the authors stress that microscopically detectable germs are frequently incapable to develop on culture media. These are frequently non-viable bacteria and in such conditions no subsequent modifications of the currently applied therapy scheme should be attempted. The authors show that only 26,2% of the germs from 519 positive cultures have developed after 21 days from the seeding. At 30 days the overall proportion increased to 71,2% while at 45 days 22,7% more positive cultures appeared and an additional 6,1% cultures became positive at 60 days. Tubercle bacilli become increasingly demanding and slower growing. If at 21 days only one colony was evident in 6% of the cases, while an infinite number of colonies were present in 58% of the cultures at 60 days a single colony was found in 42% of the cases, in contrast with only 9% cases with an infinite number of colonies (P less than 0,001). The delay in the growth of bacilli was parallel with the duration of chemotherapy. The results are compared with data from the literature.