SUMMARYDense particles (density 1.44 g/ml) were isolated during purification in CsC1 of echovirus 11 produced in HeLa cells. The dense particles had similar antigenic properties and similar RNA and protein composition to standard (density 1.33 g/ml) echovirus 11 particles. They differed from standard particles in their higher buoyant density, lower infectivity and slightly smaller diameter. In contrast to other picornavirus dense particles, echovirus 11 dense particles were present as a major component of virus population. This high ratio of dense particles was found only in virus from HeLa cells which produced non-haemagglutinating echovirus l l. Haemagglutinating echovirus i 1 from primary monkey kidney (MK) cells did not contain any detectable levels of dense particles.