Exosome fractions of dendritic cells (DC) produced in long-term cultures (LTC) were found to contain Mycoplasma contaminants. In this study, Mycoplasma-infected, -uninfected, and -reinfected cultures of DC and control cell lines have been compared for their capacity to activate lymphocytes. Using differential centrifugation, size fractionation, and inhibition assays, it has been possible to map Mycoplasma to the exosome or vesicle fraction purified from culture supernatant (CSN). Mycoplasma fractions were shown to induce proliferation of B and not T cells. The B cell response was sensitive to mitomycin C and primaquine, both known antibiotics, but resistant to protease and DNase, suggesting a role for lipoproteins. Mycoplasma-contaminated exosome fractions of LTC-DC were potent mitogens for naive B cells and promoted Ig secretion. In contrast to the polyclonal B cell mitogen LPS, they were unable to promote Ig isotype switching. They induced polyclonal activation of all B cell subsets, including naive B cells, the T1 and T2 subsets of transitional B cells, marginal zone (MZ), and follicular (FO) B cells. The B cell proliferative response was not antigen-specific and occurred independently of T cell help. Implications for autoimmune sequelae associated with Mycoplasma infection are discussed along with the possibility that primaquine could be an effective treatment for Mycoplasma infection in humans. This study highlights the close association between exosomes and infectious agents like Mycoplasma and cautions about purification procedures for preparation of exosomes for studies on immunity.