adherence of staphylococcus aureus to influenza a virusinfected madindarby canine kidney cell cultures madindarby canine kidney cells infected with influenza a virus strains pr8 fm1 jap 305 and tex 1 were tested with nine strains of staphylococcus aureus and group b streptococcus type ic to determine whether mammalian cells become susceptible to bacterial adherence as a result of virus infection bacterial adherence to virusinfected cells varied depending on the virus strain and on the strain of bacteria tested a quantitative radioassay was developed to study the parameters of adherence attachment of 3hlabeled s aureus grown in chemically defined or biologically complex medium to fm1 virusinfected cells was significantly increased p less than 00005 compared with attachment to control cells adherence coincided with the appearance of hemadsorption which is a marker of the presence of virusinduced glycoproteins on the cell surface adherence was temperature dependent increased with a decrease in hydrogen ion concentration and was not affected by the presence of k mg2 or ca2 adherence was blocked when 3hlabeled s aureus was pretreated with trypsin but not when cells were pretreated with neuraminidase