Because sarcoidosis resembles tuberculosis clinically and histologically, it has been suggested that mycobacteria might have a role in the pathogenesis of the disorder. Mycobacteria have not been found in sarcoid tissues by conventional culture techniques, so we have used a liquid-phase hybridisation method to see whether we could detect mycobacterial rRNA in such tissues. RNA was extracted from five sarcoid and five normal spleens. Extracts were assayed by liquid-phase DNA/RNA hybridisation with a DNA probe specific for the rRNA of the Mycobacterium tuberculosis complex. Hybridisation obtained with the sarcoid spleens, from which mycobacteria were neither seen on microscopy nor cultured with standard methods, was 4.8 times higher than that with normal spleens (p less than 0.001). Our demonstration of mycobacterial nucleic-acid components in sarcoid splenic tissues supports the notion that mycobacteria play a part in the cause of sarcoidosis.