Peripheral blood lymphocytes of 20 normal volunteers were separated on a discontinuous gradient of Ficoll. The ability of lymphocytes from the fractions to from sponteneous (E) and induced (EAC) rosettes and undergo transformation in the presence of phytohaemagglutinin (PHA) was tested. T-lymphocytes were concentrated in the density of 1,067 g/ml and B-lymphocytes in the density of 1,063 g/ml.