Previously, our laboratory showed that either abrupt (AW) or precipitated withdrawal (PW) from morphine led to profound suppression of murine splenic antibody responses to sheep red blood cells at 24 h post-withdrawal. In the present studies, we examined the immune mechanisms mediating suppression at that time point. A co-culture method was used to examine whether cells from withdrawn mice had (1) a deficit in function and/or (2) contained populations of suppressor cells. To examine the first hypothesis, cells from normal mice were co-cultured with cells from withdrawn mice in a 1:3 ratio (normal/withdrawn). To test the second hypothesis, the ratio was reversed. The results were paradoxical. Co-culture of cells in a 1:3 ratio showed that spleen cells from withdrawn mice had a deficit in macrophage function. Spleen cells from withdrawn mice also showed decreased mRNA levels of IL-1beta, IL-1-Ra, and TNF-alpha and a suppression of co-stimulatory molecule expression. To examine the second hypothesis, cells were co-cultured in a 3:1 ratio (normal/withdrawn). In this paradigm, spleen cells from abrupt withdrawn mice were shown to contain populations of both suppressor macrophages and B-cells. In vivo experiments carried out on mice 24 h post-withdrawal showed increased sensitivity to the lethal effects of LPS and increased production of TNF-alpha, implying a state of macrophage activation. Thus evidence for both suppressed and activated macrophages has been obtained in mice 24 h after abrupt withdrawal from morphine.