Cell proliferation is believed to contribute to the increased synthesis rate during load-induced growth of avian anterior latissimus dorsi (ALD) skeletal muscle but the relative contribution of different cell types to this proliferative response and the time-course of cell activation is not well documented. The current investigation measured the abundance and localization of cyclin A protein, which is uniquely present in proliferating cells and required for the entry of vertebrate cells into the DNA-synthesis phase, during the time-course of chicken ALD loading. Total protein content in 1.5, 7 and 13 days loaded ALD increased by 60%, 191%, and 294%, respectively. Immunoblotting analysis identified that cyclin A protein per total protein was dramatically increased in ALD muscle after 1.5 days of loading but returned to control level at 7 days. In vitro kinase assays demonstrated a corresponding massive activation of the cyclin A regulated cyclin-dependent kinase 2 (cdk2) but not of cdk2 protein level in muscle homogenates after 1.5 days of muscle loading. Immunofluorescence experiments demonstrated that the increase of cyclin A in 1.5 days loaded ALD was primarily confined to nuclei of interstitial cells (92%) but also found in fiber-associated cells (8%). In situ hybridization demonstrated an increased number of nuclei of interstitial cells expressing collagen I transcripts after 1.5 days of loading. These data show that the cell cycle protein cyclin A is induced in fiber-associated cells during the early growth response in loaded ALD but also implicate an activation of interstitial cells as playing an early role in this model for muscle growth.