The induction and the effector phase of murine delayed type hypersensitivity (DTH) were evaluated in mice treated from birth with anti-IgM antibodies; these mice had no mature B cells and could not produce an antibody response. To study the effector phase, long-term cultured cloned helper T cells were injected subcutaneously together with the specific antigen into the hind footpad of normal and B cell-deficient mice. Antigen-specific DTH responses assessed by the local swelling reaction 24 h after transfer measured against a particulate antigen (sheep red blood cells, SRBC) as well as a soluble antigen (ovalbumin, OVA) were unaffected by the absence of B cells. To study the induction phase of DTH, 3-day immune in vivo primed lymphocytes from normal or B cell-depleted mice were adoptively transferred by subcutaneous injection into the hind footpad of naive syngeneic recipients. B cell depletion did not affect the induction of cells capable of responding to SRBC; in contrast, the response to soluble antigen (OVA) was significantly reduced, suggesting that B cells or their products participated in the induction of a DTH response to a soluble antigen.