The ontogeny of Ig(+), theta(+) and "null" (Ig-, theta-) lymphocytes in the mouse spleen was followed. An event was found at about 10 days after birth when the percentage of Ig(+) lymphocytes in the spleen begins to rise sharply, and the percentage of "null" lymphocytes falls. The Ig(+) lymphocytes before and after this event were then evaluated by four criteria pertaining to B cells. Three of these, the presence of C3 receptors, "capping" behavior, and the amount and/or disposition of surface Ig distinguished the Ig(+) lymphocytes before the event described from those present afterward. The fourth criterion, the presence of Fc receptors, did not distinguish between the early and mature forms of Ig(+) lymphocytes.