A parenteral formulation of rifampicin (Rimactan® i.v., Ciba-Geigy, Basel, Switzerland) was administered to 237 critically ill or comatose patients, or patients with gastro-intestinal or absorption problems. There were 160 patients suffering from tuberculosis, 77 suffering from non-tuberculous (non-tb) infections including 30 cases of sepsis, 8 cases of bacterial meningitis and/or cerebral abscess and 9 patients with Legionnaires’ disease. The usual daily dose of rifampicin was 450–600 mg, administered in most cases by i.v. bolus (122 cases) or i.v. drip infusion (79 cases) for a period of 1–113 days. Rifampicin was in all cases combined with one or more antimicrobial drug(s). The physicians considered the therapy as successful when the treatment with oral rifampicin could be instituted soon after parenteral administration or when the patients markedly improved their clinical condition. Of a total of 123 tuberculous patients for whom assessment of efficacy was possible, 100 (81.3%) showed favourable clinical results. Of 40 non-tb patients who could be analysed for clinical progress, 32 (80.0%) had a favourable outcome. Special attention should be drawn to the 11 patients with proven staphylococcal infections, of whom 10 were cured clinically and/or bacteriologically. Thrombophlebitis occurred in 10 out of the 237 (4.2%) patients, almost always in patients who were treated for more than 30 days. Systemic unwanted effects occurred in 14 (5.9%); the relationship to the treatment was not always established. Treatment was withdrawn due to unwanted effects in 5 (2.1%) of the 237 patients. Taking into account the severe, life-threatening infections reported, the results suggest that i.v. rifampicin is useful and in some critically ill patients even life-saving. Tolerability was good, even in long-term i.v. administration, although there seems to be the possibility that thrombophlebitis might develop if treatment is continued over 3