We have investigated the ability of anti-CD28 antibody costimulation to induce resistance to macrophage (M)-tropic strains of human immunodeficiency virus type 1 (HIV-1) in vitro. Our results confirm the observations of Levine et al. (15) that stimulation of CD4 T cells with anti-CD3/anti-CD28 antibodies coimmobilized on magnetic beads renders the cells resistant to infection by M-tropic strains of HIV-1. The resistance was strongest when the beads were left in the cultures throughout the experiment. In contrast, stimulation of CD4 T cells with the same antibodies immobilized on the surface of plastic culture dishes failed to induce resistance and resulted in high levels of p24 production. This was true even if the cells were passaged continuously on freshly coated plates. If the beads were removed after initial stimulation, p24 production increased over time and produced a result intermediate to the other forms of stimulation. For beads-in, beads-out, and one-time plate stimulated cultures, resistance to infection correlated with down-regulation of CCR5 expression at the cell surface and with increased production of beta-chemokines. However, cultures of CD4 T cells continuously passaged on anti-CD3/anti-CD28-coated plates produced large amounts of p24 despite decreased levels of CCR5 expression and increasing production of beta-chemokines. Expression of the T-cell activation markers CD25 and CD69 and production of gamma interferon further supported the differences in plate versus bead stimulation. Our results explain the apparent contradiction between the ability of anti-CD28 antibody costimulation to induce resistance to HIV infection when presented on magnetic beads and the increased ability to recover virus from the cells of HIV-positive donors who are on highly active antiretroviral therapy when cells are stimulated by anti-CD3/anti-CD28 immobilized on plastic dishes.