Many Duchenne muscular dystrophy (DMD) patients are known to have rare staining dystrophin-positive fibers, termed "revertants." The precise etiology of these rare fibers is unknown. The most likely explanation, however, is somatic mosaicism or somatic reversion/suppression. Immunocytochemistry was performed on serial sections from deleted and nondeleted patients, with a panel of antibodies--9219, 1377, 9218, and Dys-2--that span dystrophin. Both familial and nonfamilial patients possessed revertants. Either the same clusters or individual revertant fibers stained with amino- and carboxyl-terminal antibodies in all 14 DMD patients. In patients with deletions, revertants did not stain with antibodies raised to polypeptide sequences within the deletion. These results indicate that positively staining fibers are not the result of somatic mosaicism in deleted patients. Five of 10 patients without deletions had revertant fibers. In two of these patients, the revertant fibers did not stain with antibody 9218, which was generated against amino acids 2305-2554 and which corresponds to exons 48-52. The remaining antibodies from the panel stained the same fibers on separate serial sections in these two patients. The most likely mechanism giving rise to these positively staining fibers is a second site in-frame deletion. Antibodies generated to polypeptide sequences within deletions can be used to control for the natural occurrence of revertant fibers in myoblast transfer studies and may be useful in the detection of point mutations.