Hepatitis B core antigen (HBcAg) expressed intracellularly in Escherichia coli. hence cell disruption process is the prerequisite step to recover this protein. The objective of this study was to develop an efficient mechanical cell disruption method (ultrasonication) for the release of HBcAg. The released intracellular protein was subsequently purified by the combination of centrifugation, precipitation. dialysis and sucrose gradient ultracentrifugation. The results show that the cell disruption and HBcAg release rates increased with the increase of acoustic power. Besides, the optimal cell suspension volume (15 ml) and sonication time (90 min) for the disruption of E. coli cells and the release of intracellular HBcAg were determined. Ultrasonication was found to be more effective than enzymatic method with respect to the release of HBcAg. HBcAg recovered from the cell lysate, resulted from ultrasonication, is 22 times higher than that of the enzymatic method. Electron microscopic analysis showed that the purified HBcAg assembled into particles that closely resemble the viral nulcleocapsid form. These particles reacted with anti-HBcAg monoclonal antibody in enzyme-linked immunosorbent assay (ELISA) showing that it is functionally active. (c) 2006 Elsevier Ltd. All rights reserved.