The bactericidal action of two therapeutic regimens on Mycobacterium tuberculosis was assessed by viable counts in serial sputum samples in 49 pulmonary tuberculosis patients being treated with rifampicin (R), ethambutol (Emb), isoniazid (I) and pyrazinamide (Z) together in a single dose thrice weekly (REmbIZ3) or with REmb and IZ on alternate days (REmb3IZ3alt). In both groups of patients, there was a significant reduction (P < or = 0.02) in the colony forming units (cfu) of M. tuberculosis per ml of sputum during the first two days of treatment itself. This early bactericidal action (EBA) as well as the reduction in counts during the subsequent days of treatment were similar (P > 0.2) for both REmbIZ3 and REmb3IZ3alt regimens indicating that splitting up REmbIZ into REmb on one day and IZ on the next day in short course chemotherapy (SCC) regimens may not affect the bactericidal action of the regimens.