Actin and tropomyosin in muscle samples from normal humans, from human fetuses between 12 and 17 gestational weeks, and from patients with a variety of neuromuscular disorders were studied with two-dimensional electrophoresis using isoelectric focusing with either a broad pH range (8.6-4.5) or a narrow pH range (5.9-3.8) for the first dimension and either SDS or SDS-urea for the second dimension. With the broad pH range, two brothers with Duchenne muscular dystrophy were noted to have a less acidic variant of aL-tropomyosin in biceps muscle which was not found in biceps muscle from other patients or controls. Studies of 8 additional biopsy specimens from patients with Duchenne muscular dystrophy and comparison with both fetal and normal human muscle using the narrow pH range revealed multiple forms of actin and tropomyosin which varied from individual t o individual. This heterogeneity appeared to be unrelated to the dystrophic state but also obscured the ability to detect a change in actin or tropomyosin which could be related to dystrophy.