Dystrophin is the gene product which is affected in Duchenne muscular dystrophy (DMD). We studied differentiating clonal muscle cultures derived from normal muscle and from the mother of a DMD patient by immunocytochemistry, using anti-dystrophin antibody. While clonal cultures derived from normal muscle expressed dystrophin in all myotubes, two populations of myogenic cells could be demonstrated in muscle from this possible DMD carrier; in 13 clones the myotubes expressed dystrophin and in 7 clones dystrophin was undetectable. No DNA deletion, duplication or rearrangement was detected by Southern blot analysis of DNA from this family using cDNA probes. Thus, immunocytochemical analysis of clonal muscle cultures may be a useful method to determine whether mothers of DMD patients are carriers of the DMD mutation, especially in the absence of demonstrable gene defects.