The role of somatic division in secondary anti-BSA responses by isolated rabbit cells was investigated with the use of tritiated thymidine as a marker in combination with Coon's immuno-fluorescent technique and colchicine, a mitotic inhibitor. The cells were cultured in 0.1-µ porosity diffusion chambers implanted into x-irradiated rabbits and mice. The results showed that not only were antibody-containing cells dividing but that they were dividing at a rate significantly higher than that of the incompetent cells during the log phase of activity. The mean generation time was 12 hr during the log phase and 24 hr in the stationary phase. A direct correspondence was found between the rise in percentage of antibody-containing cells and rise in antibody titer of culture chamber fluid. We also observed that all the competent cells of the stationary phase were derived from precursor cells through somatic division. Cytomorphologic examinations of antibody-containing cells at intervals after antigenic stimulation suggest a gradual transition in the number and distribution of sites of antibody synthesis.