Protective immunity in paracoccidioidomycosis is mainly mediated by cellular immunity. The role of B cells in this disease, in particular B-1 cells, is poorly understood. The aim of this study was to characterize the participation of B-1 cells in resistance or susceptibility of BALB/c and BALB/Xid mice to P. brasiliensis (Pb) pulmonary infection. BALB/Xid, which lacks B-1 cells, exhibited higher resistance to infection when compared with BALB/c mice. However, adoptive transfer of B-1 cells to BALB/Xid mice drastically increased the susceptibility of these animals to Pb infection. The fungal burden in BALB/c and B-1-reconstituted BALB/Xid was significantly higher as compared to BALB/Xid strain. Compact, well-organized granulomas were observed in the lungs of BALB/Xid mice, whereas large lesions with necrotic center with a plethora of fungi developed in BALB/c mice. It was also shown that B-1 cells impair phagocytosis of Pb by macrophages in vitro via secretion of IL-10, which was increased upon stimulation with a purified Pb antigen, gp43. Finally, in vivo blockade of IL-10 led to a better control of infection by the highly susceptible B10.A mouse. These findings suggest that B-1 cells play a major role in resistance/susceptibility to Pb infection in murine models, most likely via production of IL-10.