This study focuses on the effect of varying regimens of cocaine administration on three parameters of the immune response: antibody production, resistance to infection by Streptococcus pneumoniae following immunization, and resistance to tumors. The effect of cocaine on antibody production of female and male BALB/c mice was investigated to both a T-independent (pneumococcal polysaccharide type III [SSS-III]) and a T-dependent antigen (the 2,4-dinitrophenyl ligand [DNP]). It was found that high doses of cocaine injected 3 times/day prior to SSS-III resulted in a small rise in antibody levels in male mice. Low doses given for 4 days prior to or subsequent to SSS-III injection had no effect on the antibody response nor on the susceptibility to infection by live S. pneumoniae. High dosages of cocaine administered 3-5 times/day had no effect on the anti-DNP immune response of male mice but resulted in an almost 2-fold increase of anti-DNP plaque-forming cells in female mice.