Freshly isolated KCs (panels A, C, E) or J774E cells (panels B, D, F) were seeded into 96-well microtiter plates and then treated with ricin plus indicated amount of anti-RTA MAbs for 18 h. The cells were washed, and cell viability was assessed 24 h later. The MAbs are grouped based on their epitopes clusters on RTA: cluster I (A, B), cluster II (C, D) and Clusters III and IV (E, F). Each data point represents the average (with corresponding standard deviations) of three independent experiments.