A thermodynamic approach has been used to measure the amount of haemagglutinin and matrix protein expressed at the surface of P815 cells infected for periods between 4.5 and 11 h with either WSN (H0N1) or JAP (H2N2) strains of type A influenza virus. This involved measuring the interaction of different concentrations of labelled (Fab)2 preparations of specific antibody with normal and infected cells. Assuming that one molecule of (Fab)2 bound to one molecule of antigen, values for the number of molecules of antigen/infected cell ranged from 7.6 X 10(5) to 1.7 X 10(7) for haemagglutinin and 1.3 X 10(5) to 1.1 X 10(6) for matrix protein. The ratio of haemagglutinin/matrix protein was lower for WSN-infected cells (1.7) than for JAP-infected cells (10). The same reagents were reacted with three purified A type virions; WSN, JAP and Port Chalmers (H3N2). Each preparation bound anti-matrix protein (Fab)2 though the value for haemagglutinin/matrix protein was much higher (66) than for infected cells and suggested that a virion may have a small number (about 12) of matrix protein molecules exposed though it was not excluded that the matrix protein detected was exposed only on damaged virions. Pre-treatment of infected cells with unlabelled reagent (anti-haemagglutinin) reduced the subsequent binding of the same labelled reagent but not the binding of the labelled matrix protein reagent and vice versa, suggesting that the haemagglutinin and matrix protein were not very close to each other on the cell surface.