The matrix protein of influenza virus is a major structural component of the virion which is generally believed to bridge between the membrane envelope and the ribonucleocapsid core. To investigate the interaction of M1 with cellular membranes in the absence of other influenza proteins, we examined its distribution by subcellular fractionation after expression in HeLa cells. Approximately 81 to 88% of M1 protein, expressed without other viral proteins, was soluble, whereas the remaining 12 to 19% was tightly associated with membranes. Conditions known to release peripherally associated membrane proteins did not detach M1 proteins from isolated membranes, suggesting that the fraction of M1 bound to membranes behaves as an integral protein. Coexpression of M1 with hemagglutinin or neuraminidase did not alter the extent of membrane association of M1 protein, indicating that there is no strong interaction between M1 and the cytoplasmic tails of the viral glycoproteins. Additional attempts were made to identify membrane binding domains in M1 protein. Mutants constructed with mutations in the four hydrophobic regions thought to be responsible for membrane association still exhibited the same levels of membrane association as that observed with wild-type matrix protein. Therefore, specific hydrophobic domains are apparently not required for membrane binding.