Mice were primed by footpad inoculation of small amounts of BSA (0.1–2.5 μg) in incomplete Freund's adjuvant. Lymphoid cells removed from these mice 50–120 days after priming were cultured in diffusion chambers, either in the presence of BSA or following a brief period of incubation with BSA in vitro. Small but clearly measurable amounts of anti-BSA antibody were produced by cells stimulated with amounts of antigen in the range 0.1–10 μg. The optimum boosting dose seemed to be related to the priming dose. The system described appears to be eminently suitable for the autoradiographic follow up of the interaction of cells with antigen in the secondary response.