The techniques of cost-effectiveness are employed to simplify resource allocation decisions concerned with the use of land and air transport systems in the health services of a developing country. Outcome classifications are produced for patients seen by mobile and fixed primary care units. The mobile services examined were far more costly (8 to 14 times greater) per likely-effective-patient-contact than comparable care delivered from permanently staffed fixed clinics. This was particularly so for the air-delivered service. The disparity in cost-effectiveness was due mainly to the small proportion of patients seen by the mobile services who could be treated effectively in contrast to a far larger proportion at fixed clinics. This was a consequence of the periodic availability of care from the mobile services as against the continuing provision of care at fixed clinics. The main justification for the use of mechanical transport in connection with primary health care is regular supportive (not policing) visits by skilled health workers to rural clinics. Land vehicles are cheaper than aircraft for visiting the more accessible facilities; for the more distant clinics, the cost of journeys by land vehicle are similar to those by aircraft.