Three parameters allowing to distinguish B and T lymphocyte populations, i.e., the presence of surface-associated immunoglobulins (slg), the electrophoretic mobility (EPM) and the responsiveness in vitro to bacterial lipopolysaccharide (LPS) and to concanavalin A (Con A), were studied on mouse spleen cells at various time intervals following a single injection of cyclophosphamide (300mg/kg body weight). 1 day after the treatment, B cells (sIg-bearing, slow EPM) were almost totally destroyed and likewise, the response to LPS was abolished. Regeneration of the B cell compartment did not begin before day 16 and was complete only by day 30. Reconstitution of the LPS responsiveness was found to parallel the reappearance of B cells. T cells were also affected by the treatment, although to a much lesser extent than B cells. A first T cell subpopulation, with a mean EMP = 1.10 μ msec––1 V––1 cm, was destroyed by cyclophosphamide, while a large part of the second T cell subpopulation, with a mean EPM = 1.25 μ m sec––1 V––1 cm remained in the spleen. This strongly suggests that the former subpopulation probably corresponds to short-lived cells and the latter to long-lived ones. Con A responsiveness was diminished but never suppressed and recovered relatively early. Starting by day 8, complete regeneration of the T cell compartment was achieved by day 30.