Clone 33D1 is a mouse-rat hybridoma that secretes a specific anti-dendritic cell (DC) monoclonal antibody (14). Because the antibody kills DC in the presence of rabbit complement, it can be used to study the functional consequences of selective DC depletion. Previous data on the cell specificity of 33D1 were first extended. By cytotoxicity (rabbit complement) and indirect immunofluorescence (biotin-avidin technique), 33D1 reacted with DC but not with macrophages nor other splenocytes. In contrast, the monoclonal antibody, F4/80 (15), reacted with macrophages but not DC. The functional assay evaluated in this paper was stimulation of the primary mixed leukocyte reaction (MLR). 33D1 antibody itself did not inhibit stimulation by enriched populations of DC. In the presence of complement, 33D1 killed DC and ablated stimulatory function. The effect of 33D1 and complement on MLR stimulation by heterogenous cell mixtures was then evaluated. Removal of DC from unfractionated spleen suspensions reduced stimulatory capacity 75-90 percent, comparable to that produced with specific anti-Ia antibody and complement. Stimulation of both proliferative and cytotoxic responses was reduced. DC depletion had similar effects on MLR generated across full strain differences, or across selected subregions (H2I, H-2K/D) of the major histocompatibility complex. To further compare the functional properties of spleen DC and macrophages, MLR stimulation by adherent and nonadherent fractions of spleen were tested separately. 62 +/- 8 percent of the total stimulatory capacity of spleen was in the plastic adherent population. Activity was ablated greater than 90 percent after elimination of DC. MLR stimulation by 24-h cultures of spleen adherent cells, which contained a three- to sixfold excess of Ia(+) macrophages, was also ablated when DC were removed. Stimulation by nonadherent spleen was more resistant, but was reduced 50-75 percent by 33D1 and complement. The function of spleen cells treated with 33D1 or anti-Ia antibody and complement was restored with a small inoculum of purified DC. The latter corresponded to 0.5 percent of total stimulator cells and were enriched by previously described techniques that did not require the 33D1 antibody. We conclude that the DC, a trace component of mouse spleen, is the principal cell type required for stimulation of the primary MLR. Because other cells are not immunogenic, but do express Ia and H-2 alloantigens, DC likely represent the critical accessory cell required for the induction of lymphocyte responses.