The effect of inactivated phase I and phase II Coxiella burnetii whole cell vaccine (WCV) on the response of murine spleen cells to mitogenic and antigenic stimuli was evaluated in C57BL/10 ScN endotoxin nonresponder mice with an in vitro lymphocyte proliferation assay. Intraperitoneal injection of phase I WCV into mice resulted in marked and persistent suppression of the proliferative response of spleen cells to concanavalin A, phytohemagglutinin, and pokeweed mitogen. This response was time and dose dependent and was not associated with decreased lymphocyte viability. By using a standard dose of 100 ,ug of phase I WCV, suppression of mitogenic responsiveness was first detected 3 days postinjection, attained maximum levels by day 14, and persisted for longer than 5 weeks. Suppression of mitogenic lymphocyte proliferation also was demonstrated after inoculation of animals with viable phase I organisms. The observed hyporesponsiveness of spleen cells from phase I WCV-injected animals was not either the result of a shift in the mitogenic dose optimum or due to a change in the day of in vitro peak response. Spleen cells from phase I WCV-injected mice were negatively regulated with homologous antigen. Investigation of the mechanism of action of phase I WCV, with a 51Cr-release assay, and trypan blue dye exclusion showed that phase I WCV was not directly cytolytic or cytotoxic to spleen cells from normal or vaccinated mice. Phase II WCV did not induce significant mitogenic hyporesponsiveness or negative modulation of spleen cells. These findings extend the observations of adverse host responses associated with the phase I WCV and underscore the need to develop a microbial fraction which possesses protective potency but which lacks the immunosuppression.propensity to induce deleterious tissue reactions and Vaccination against Q fever in humans was introduced in 1948 by Smadel et al. (39). The whole cell vaccine (WCV) consisted of a formalin-inactivated suspension of Coxiella burnetii prepared from infected yolk sacs. The protective value of phase I WCV has been well documented (19,28,30,43). Despite the prophylactic efficacy of phase I WCV, we are concerned about the potential for adverse reactions induced by phase I WCV (45). When phase I WCV was first developed and tested, Smadel et al. observed local skin reactions, fever, anorexia, and malaise in vaccinated individuals (39). Since then, various severe local reactions (28) and occasional systemic reactions (45) have been described. Recently, phase I WCV administered intraperitoneally has been shown to induce splenomegaly, hepatomegaly, liver necrosis, and anemia in C57BL/10 ScN mice (45). The undesirable effects of phase I WCV have been severe enough to preclude its widespread and unconditional use in humans (1).The effect of the phase I C. burnetii WCV in mice has been defined largely in terms of gross pathological changes. In a previous study, the subject of host cellular immunological functions was broached and the occurrence of mitogenic hyporesponsiveness in C5...