In this study we investigated the in vitro mitogenic properties of the capsular carbohydrate of Hemophilus influenzae b, polyribosylribitolphosphate (PRP). PRP was found to be a potent polyclonal activator of murine B lymphocytes. PRP induced normal B cells to undergo blastogenesis, DNA synthesis, and differentiation to IgM and IgG secretion. IgG3 accounted for the majority of the IgG. No PRP-specific antibody was detectable, indicating the polyclonal origin of the secreted immunoglobulin (Ig). T lymphocytes were neither activated by PRP nor required for B cell proliferation or Ig secretion. In addition, T cell-depleted spleen cells also depleted of accessory (A) cells by passage through Sephadex G-10 retained responsiveness to PRP. Trace lipopolysaccharide (LPS) contamination was not responsible for the mitogenic effect, as shown by the ability of C3H/HeJ spleen cells to proliferate in response to PRP and by the failure of polymyxin B to inhibit PRP-induced DNA synthesis. The B cell responses induced by PRP and LPS were similar with respect to T cell and A cell independence, to the magnitude of DNA synthesis, and to Ig secretion and the Ig isotypes expressed. These data, taken with the finding that the combination of optimal doses of PRP and LPS did not give an additive DNA synthetic response, indicate that PRP and LPS were activating similar B cell populations. However, in contrast to LPS, PRP was capable of inducing significant DNA synthesis in cultures containing as few as 1,000 B cells, suggesting that PRP-driven proliferation was less dependent on cellular interactions than the response to LPS. The differential ability of PRP and LPS to stimulate C3H/HeJ B cells and to stimulate B cell proliferation at low density indicates basic differences between these two mitogens in their mechanisms of B cell activation.