CD28 activation inhibits myeloma cell proliferation. 8226, U266, and MM.1S (2 × 10⁵) were cocultured with uncoated beads (beads alone) or 9.3-coated beads. K562 (CD28⁻ control; 2 × 10⁴) was cultured in media alone or 9.3-coated beads. After 24 hours, cells were pulsed with ³H TdR, and incorporation was measured 18 hours later and expressed as means ± SD of triplicate wells. Data shown are 1 experiment representative of 6 independent experiments. Comparing the proliferation between the control versus 9.3-coated beads: P = .025 for 8226, P = .003 for U266, P = .001 for U266, and P = .82 for K562.