Graded numbers of marrow cells and 5 x 10(7) thymocytes were mixed in vitro and transplanted into X-irradiated (C3H x C57BL/10)F(1) mice. Upon injection of sheep or chicken erythrocytes, splenic plaque-forming cells secreting IgM (direct PFC) or IgG (indirect PFC) hemolytic antibody were enumerated at the time of peak responses. Anti-sheep and anti-chicken primary PFC responses elicited by nonimmune marrow cells differed sharply from each other under the conditions of limiting dilution assays. The frequencies of anti-chicken responses in recipients of different numbers of marrow cells conformed to the predictions of the Poisson model, while the frequencies of anti-sheep responses did not. Hence, the function of certain marrow-derived cells was expressed differentially during the two immune responses, to exclude that the same precursor units generated anti-sheep or anti-chicken PFC. The former precursor cells or units were functionally more heterogeneous than the latter. Immunization of marrow donors against sheep erythrocytes did not alter the population of cells engaged in anti-chicken responses, since limiting dilution assays with immune and nonimmune marrow cells gave identical results. However, anti-sheep immunization altered specifically the cell population engaged in anti-sheep responses, in two ways: (a) potentially immunocompetent marrow cells underwent antigen-dependent differentiation or maturation, to become functionally homogeneous. Consequently, the frequencies of PFC responses in limiting dilution assays conformed to the Poisson model; the changes occurred independently in class-restricted precursors of direct and indirect PFC. (b) marrow cells capable of inhibiting precursors of direct anti-sheep PFC arose in primed mice. The inhibition, which was specific, could have been effected directly by marrow cells or by a diffusable product such as IgG antibody. Results indicated that potentially immunocompetent cells of mouse marrow with distinct functions were antigen specific and antigen sensitive.