In polymyositis (PM) and inclusion body myositis (IBM), but not in dermatomyositis there is evidence of cell-mediated cytotoxicity: T8+ cells accompanied by macrophages focally surround, invade, and destroy nonnecrotic muscle fibers. However, the T8 marker appears on both cytotoxic (Tc) and suppressor (Ts) cells. The Leu-15 marker appears on Ts but not on Tc cells, but it also appears on macrophages and on some killer/natural killer cells. To obviate this problem, the T8, Leu-15, and Leu-7 markers were demonstrated by sequential paired immunofluorescence in single cryostat sections. Using this approach, we reliably differentiated for the first time between Tc and Ts cells in tissue sections. Six cell phenotypes were identified: T8+ Leu-15-Leu7- Tc cells, T8+ Leu-15+ Leu-7- Ts cells, three types of Leu-7+ killer/natural killer cells, and T8-Leu-15+ Leu-7- macrophages. Muscle specimens from 5 patients with PM and 5 with IBM were studied. In each case, 6 nonnecrotic muscle fibers focally surrounded and invaded by mononuclear cells were selected randomly. A total of 2,022 mononuclear cells were analyzed, 870 from patients with PM and 1,152 from those with IBM. When counts of the identified cell phenotypes in individual patients were pooled, there were four times as many T8+ Leu-15- Leu-7- Tc cells as T8+ Leu-15+ Leu-7- Ts cells in either PM or IBM samples. However, when the relative frequencies of the Tc and Ts cells were examined in individual patients, the Tc cells tended to become more abundant, and the Ts cells correspondingly less abundant, with the duration of symptoms.(ABSTRACT TRUNCATED AT 250 WORDS)