The main study of the work reported concerned the interaction of Adenovirus type 5 penton antigen with HeLa cells. It was found that penton antigen inhibited the spreading of HeLa cells on glass and this effect, together with the Early Cytopathic Effect or Toxic Effect of the antigen. was used as an indicator of the antigen interaction with cells. Preparations of penton antigen were obtained by fractionation of adenovirus type 5 harvests on diethylaminoethyl Sephedex. It was shown that the inhibition of cell spreading was specific to the penton antigen and that inhibitory activity resided in the base of the antigen particle. as does activity leading to the Early Cytopathic Effect. Two techniques which determined the inhibition of spreading of single HeLa cells were developed and these were used for quantitative and other studies on the antigen/cell interaction. The inhibition of spreading of single Meta cells by antigen as found to follow first order kinetics. and the dose/response relationship indicated that antigen particles acted singly without cooperation. Kinetic and adsorption experiments showed that little or no antigen activity was lost In bringing about the inhibition of spreading. It was found that attachment of antigen and the development of' resistance to the protective effect of antiserum and trypsin by antigen/ cell complexes were necessary for inhibition of spreading to take place. iii These processes also followed first order kinetics and were uninfluenced by treatment of cells with Sodium fluorides it was suggested that the rates of inhibition and of these intermediate stages ware governed directly by the rate of attachment of antigen to cells. Inhibition of spreading was temperature dependent but not dependent on cellular mecromolecular synthesis. Penton antigen did not affect cellular protein synthesis, cellular giycolysis, or gross functions of the cell membrane. It was confirmed that cells recovered from inhibition of spreading It was concluded that the penton antigen of adenovirus type 5 brought about the inhibition of cell spreading and the Early Cytopathic Effect by cell changes independent of the organisation of the cell responsible for macromolecular synthesis and of membrane function. and that at least some of the processes leading to these cell changes take place at the cell surface.