We have identified an open reading frame, EP402R, within the EcoRI E' fragment of the African swine fever virus genome that encodes a polypeptide of 402 amino acid residues homologous to the adhesion receptor of T cells, CD2. Transcription of EP402R takes place during the late phase of virus replication. The disruption of EP402R, achieved through the replacement of a 354-bp-long fragment from within EP402R by the marker gene lacZ, does not affect the virus growth rate in vitro but abrogates the ability of the virus to induce the adsorption of pig erythrocytes to the surface of infected cells. This result demonstrates that the protein encoded by EP402R is directly involved in the hemadsorption phenomenon induced by the infection of susceptible cells with African swine fever virus.