This unit describes the antigenic stimulation of in vitro antibody production by B cells and the subsequent measurement of secreted antibodies. A generalized system for inducing in vitro antibody production is presented along with a procedure for quantifying the number of antibody-producing cells by plaque-forming cell (PFC) assays: the Cunningham-Szenberg technique and the Jerne-Nordin technique. The assay can be modified as described to measure all classes of antibodies or to enumerate total immunoglobulin-secreting B cells. A protocol for preparing the resting B cells by Percoll gradient centrifugation is also described.