Some membrane and functional properties of lymphocytes have been examined in a group of children affected with persistent hepatitis. Both absolute and percentage values of B lymphocytes (cells bearing surface Ig) were normal, whereas T lymphocytes (E rosette technique) were decreased in percentage but not as absolute numbers. Only in a close range of mitogen concentration PHA responsiveness was altered, and no modification was found in the percentage of EA and EAC rosettes. A marked increase in lymphocytes lacking both T and B cell markers was observed, which may be related to an increase in precursors of mature cells or to a masking of receptors on the membrane of lymphocytes, but which is probably independent of an increase in the number of K cells, since cells bearing receptors for the Fc fragment were normal.