Comparisons of samples positive for mitochondrial DNA (mtDNA) (based on PCR fragment intensities on SDS–polyacrylamide-gel electrophoresis [SDS–PAGE]) in plasma and synovial fluid (SF) samples both from patients with rheumatoid arthritis (RA) and from controls. The arbitrary cutoff threshold is based on comparisons between the intensities of PCR-amplified mtDNA fragments and the 676-bp band on each SDS–PAGE (see Fig. 1). Data points above the cutoff threshold indicate mtDNA-positive samples, and data points below the cutoff threshold indicate mtDNA-negative samples. Correlations of the relative band intensities between plasma and SF samples from individual patients are indicated by lines joining the corresponding data points. The samples from patients with RA (n = 54) are divided into four categories (see the results section): I, PL+/SF+; II, PL+/SF-; III, PL-/SF+; IV, PL-/SF-. The control samples were from two different groups of subjects. The control plasma (PL) samples were obtained from healthy volunteers, and the control SF samples were recovered at autopsy from non-arthritic patients.