It has been suggested that the technical complexities, the expense of equipment and consumables, and problems associated with contamination make the polymerase chain reaction (PCR) inappropriate for use in developing countries. These problems were addressed using a novel one-tube nested PCR, small reaction volumes and a 'three room' system for the detection of Mycobacterium tuberculosis. The PCR of sputum samples dried on small filter paper disks was also investigated. Using this strategy 5 smear-positive and 15 smear-negative specimens were correctly identified by PCR. This method of sample collection has the advantage that samples can be sent by post and stored in a minimum of space, and remain viable for PCR for at least 4 years after collection. These and future modifications to the PCR protocol will make the assay more suitable for use in the tropics.