A novel three-dimensional (3D) skeletal muscle model composed of C2C12 mouse myoblasts is described. This model was generated by cultivating myoblasts in suspension using the rotary cell culture system (RCCS), a unique culture environment. Single-cell suspensions of myoblasts were seeded at 5 x 10(5)/ml in growth medium without exogenous support structures or substrates. Cell aggregation occurred in both RCCS and suspension control (SC) conditions within 12 h but occurred more rapidly in the SC at all time intervals examined. RCCS-cultured myoblasts fused and differentiated into a 3D construct without serum deprivation or alterations. Syncitia were quantified at 3 and 6+ d in stained thin sections. A significantly greater number of syncitia was found at 6+ d in the RCCS cultures compared to the SC. The majority of syncitia were localized to the periphery of the cell constructs for all treatments. The expression of sarcomeric myosin heavy chain (MHC) was localized at or near the periphery of the 3D construct. The majority of MHC was associated with the large cells (syncitia) of the 6+-d aggregates. These results show, for the first time, that myoblasts form syncitia and express MHC in the presence of growth factors and without the use of exogenous supports or substrates. This model test system is useful for investigating initial cell binding, myoblast fusion and syncitia formation, and differentiation processes.