An enterovirus (EVU-16) previously isolated from children with acute infectious lymphocytosis has been further characterized. The EVU-16 virus sediments as a 135S particle in sucrose gradients, has a density of 1-335 g/ml in CsCl, contains 4 polypeptides and has a single stranded RNA genome sedimenting at 35S. These structural features as well as the presence of a virus-related particle, the procapsid, are similar to those of other enteroviruses. However, the largest polypeptide of EVU-16 is 49000 daltons, which is considerably larger than the corresponding polypeptide from poliovirus; the sizes of the other three viral polypeptides were similar in both viruses. Attempts to induce lymphocytosis by the inoculation of EVU-16 into various animals, including immunologically aberrant 'nude' mice, were unsuccessful.