Five 4-mo regimens of chemotherapy for tuberculosis are compared. The two regimens in which rifampicin was given throughout the 4 mo were associated with bacteriological-relapse rates of 8% in the first 6 mo after stopping chemotherapy, but the three regimens in which rifampicin was given for only the first 2 mo had relapse-rates of 24-32%. There was no evidence that the addition of pyrazinamide in the second 2 mo of chemotherapy reduced the bacteriological-relapse rate. Removal of the streptomycin from the first 2 mo appeared to reduce the bactericidal and sterilising activity of the regimen, although the differences were not statistically significant. The incidence of adverse reactions was very low with all five regimens.