Marginal zone B cells influence spleen cell tumor lytic capacity. The influence of marginal zone (MZ) B cells on tumor lysis was determined by comparing the percentage lysis of MADB106 tumor cells in the presence or absence of MZ B cells. Total lymphocytes or lymphocytes depleted of MZ B cells from spleen were incubated with viable fluorescently-labeled MADB106 tumor cells for 90 min. Tumor lysis was measured by propidium iodine (PI) staining of labeled MADB106 tumor cells. Percentage lysis was expressed as the percentage of fluorescent positive tumor cells staining positive for PI in the presence of lymphocyte cultures minus the percentage of PI-labeled tumor cells without lymphocyte cultures. The data shown were analyzed by analysis of variance (ANOVA). Data is expressed as the mean ± S.E. (n = 3) at a given effector:target (E:T) ratio. Asterisks indicate statistical differences (P less than or equal to 0.05) compared between total and MZ B cell-depleted groups at each E:T ratio.