Dane particles were isolated in a large scale from plasma of asymptomatic carriers of hepatitis B surface antigen. The core of Dane particles was exposed by treatment with NP-40 and 2-mercaptoethanol. The antigenicity of hepatitis B e antigen (HBeAg) was not detected on the surface of Dane particles or of their cores. However, when Dane particle cores were treated with Pronase, some activity of HBeAg was liberated from them as a small molecule. When the cores were further treated with SDS, they revealed a high activity of HBeAg, indicating that HBeAg existed in the core of Dane particles in a cryptic form, which can be exposed by treatment with proteolytic enzyme or SDS. Dane particles and their cores were subjected to polyacrylamide gel electrophoresis in SDS. After electrophoresis, the gel was cut into two halves, and each half was determined for polypeptide composition and for HBeAg activity. Both of Dane particles and their cores disclosed two peaks of HBeAg activity associated with molecules with a size of 19,000 and 45,000 daltons. These two polypeptides were the major constituents of the cores. Dane particles revealed several polypeptides in addition to the HBeAg polypeptides, which were identified as hepatitis B surface antigen components. On the basis of the results obtained, HBeAg is an integral component of Dane particles, the presently accepted hepatitis B virions. These results provide the basis for the close correlation between HBeAg and Dane particles in the serum of persons infected with hepatitis B virus.