To determine if membrane association is an intrinsic property of the influenza virus matrix protein (M1) it was expressed from cDNA in living cells in the absence of other influenza virus proteins. By using a membrane fractionation scheme the M1 protein was found to associate with membranes in a time-dependent manner (0 time = 45% total; after a 3-hr chase period = 68% total M1 protein). Coexpression of the integral membrane proteins HA+NA+M2 did not significantly increase the association of the M1 protein with cellular membranes, indicating that putative interactions of the M1 protein and the cytoplasmic tails of the integral membranes cannot be detected by this assay. Biochemical treatments of the M1 protein associated with membranes with alkali, high salt conditions, or Triton X-114 yielded data that challenge the normal criteria for integral membrane proteins or peripheral membrane proteins. Examination of the solubility of the M1 protein in influenza virus-infected cells to Triton X-100 extraction indicated it became increasingly insoluble with time, but the M1 protein could be solubilized in Triton X-100 containing 1 M NaCl, suggesting an association of the M1 protein with the cytoskeleton. However, when the M1 protein was expressed from cDNA, it did not become insoluble to Triton X-100 extraction, suggesting an interaction of the M1 protein unique to the influenza virus-infected cell.