To evaluate the effects of acute cocaine withdrawal on the immune system of pregnant women, we analyzed changes in a panel of cell surface markers and plasma proteins that have immunological importance. The cell surface markers included complement receptors [CR1 (CD35), CR2 (CD21), CR3 (CD11b, CD18)], immunoglobulin Fc receptors [FcgammaRII (CD32), FcgammaRIII (CD16)], proteins important for lymphocyte function [CD3, CD4, CD8, CD19, CD25, CD45RA], and the framework antigen HLA-ABC. We also measured levels of the plasma proteins C3, C4, IgG, IgM, and IgA, along with the cytokine interleukin-2, soluble lymphocyte markers sCD25, sCD4, sCD8, IL-2, and soluble immune complexes. While no significant changes were seen in the levels of plasma proteins, changes paralleling the course of acute withdrawal were seen in complement receptors and immunoglobulin Fc receptors on leukocyte subpopulations. By contrast, proteins important for lymphocyte function were relatively unperturbed. There was an increase in receptor expression at the onset of withdrawal that peaked 3-5 days after last cocaine use, followed by a decrease in expression to initial (pre-withdrawal) levels. These changes in cell surface receptors may reflect altered immune function in the women who were withdrawing from cocaine.