Dense, RNase-sensitive, VP2-containing, non-infectious hepatitis A virus (HAV) particles were found to be formed at early times after the infection of cultured cells. These particles formed with kinetics mirroring those reported for HAV uncoating, The kinetics of the formation of dense HAV particles corresponded to a decrease in detectable, mature input virions, as detected by RNA dot blot hybridization of CsCl density gradient fractions. The dense HAV particles did not appear to have altered sedimentation coefficients, a nd as the fate of sm all capsid protein VP4 is not yet known, these particles cannot yet be termed 'A particles' or 'infectosomes', as have the uncoating intermediates in some picornavirus-cell systems. Copyright (C) 2000 S. Karger AG, Basel.