Foot-and-mouth disease virus (FMDV) attached to pig kidney cells at 0 degrees C and could only be recovered in a form with a sedimentation coefficient and buoyant density lower than that of the native virus. Incubation of the virus-cell complex at 37 degrees C caused disruption of about 80% of the particles into a 12S protein sub-unit that had the same polypeptide composition as that produced by reducing the pH of the virus below pH7. The remaining 20% had the same polypeptide and RNA composition as the native virus but it had a lower sedimentation coefficient, buoyant density and specific infectivity. These lower values are probably due to the association of the virus with cell membrane components. The 12S subunits were shown to be located inside the cell, indicating that disruption of the virus had occurred within the cell. The results are discussed in relation to the different cell mediated alteration of other picornaviruses.