Three restriction points control the cell cycle of activated murine B lymphocytes in a synergistic way. The first is controlled by the occupancy of surface immunoglobulin either by antigen- or by immunoglobulin-specific antibodies. The second is controlled by the complement C3d receptor CR2 which can be occupied by cross-linked C3b or C3d to stimulate the entry into S phase, or by soluble C3d or a C3 alpha-chain peptide, binding to the CR2 receptor, which inhibit the entry into S phase. Macrophages produce so-called alpha factors which also control the B-cell cycle at the same point. Thus, it is suspected that macrophages produce components of the early pathway of complement activation which finally lead to cross-linking of CR2 receptors on B cells. The third restriction point is controlled by unknown receptors that recognize so-called beta factors produced by helper T lymphocytes.