Entry of Epstein-Barr virus into human lymphoblastoid cells (Daudi cells) was studied by electron microscopy. At the site of viral attachment, two distinct interactions conducive to penetration of the virus occurred between the viral envelope and cell membrane, namely, (i) simultaneous dissolution of both the envelope and cell membrane, presumably resulting in passage of viral capsids into the cytoplasm and (ii) dissolution confined to the cell membrane with resulting penetration of enveloped virus. In the latter case envelope dissolution appears to occur subsequently in the cytoplasm with release of capsids. Fusion of the viral envelope with the cell membrane was not observed. The capsids exhibited two distinct structural forms--one dense, the other translucent or light in appearance. The former disrupted near the cell membrane with release of viral cores into the cytoplasm whereas the light capsids containing dense cores migrated toward the nucleus and accumulated in the perinuclear region. Apparently the process of releasing deoxyribonucleic acid (DNA) from the light capsid is slowed down or prevented in Daudi cells. A hypothesis is presented concerning the manner in which these two types of capsids initiate infection.