During myogenesis in vitro, the amount of gelsolin in myogenic cells increased by a factor of approximately 3 from about 200 ng to a maximum of 750 ng per milligram of total protein. Gelsolin increased steadily from the myoblast state to terminally differentiated myotubes containing abundant cross-striated myofibrils. At the same time, the amount of total actin varied by only about 30%, the molar ratio of gelsolin:actin increased from 1:500 to approximately 1:150. This modulation of gelsolin expression was observed both in avian and mammalian myocultures. Once the state of terminal differentiation in myocultures was attained, the amount of gelsolin decreased again. On the other hand, gelsolin decreased continuously in the postnatal mouse muscle by a factor of approximately 5 between Day 1 and Day 12 after birth. When myogenic cells from various stages of differentiation were extracted with Triton X-100, the majority of gelsolin was soluble, whereas a minor fraction was tightly associated with the cytoskeleton. The actual amount of insoluble gelsolin depended on both the Ca2+ concentration during extraction and the degree of differentiation. Whereas at [Ca2+] > 10(-5) M about one-third of the total gelsolin was associated with the cytoskeleton at all stages of differentiation, the amount of insoluble gelsolin after Triton extraction in the presence of EGTA increased from 3 to 17% during differentiation. The amount of soluble actin decreased from 40 to 25% during the same period, independent of the Ca2+ concentration. We calculated that the amount of gelsolin associated with the cytoskeletal or myofibrillar system is approximately 20-fold higher in differentiated myotubes than in early myotubes, indicating a functional role of gelsolin for myofibrillar assembly.