Castor bean proteins were separated and identified by isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis and blotted onto nitrocellulose paper. The capacity of the castor bean proteins to bind human IgE was probed with sera from castor bean-sensitive patients and radiolabeled anti-IgE. It proved difficult to identify allergens with isoelectric focusing. However, three allergens were identified when proteins were first separated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis: the 2S storage albumin, the 11S crystalloid proteins, and a third protein doublet with molecular weights of 47 and 51 kd. Specific IgE antibody to the 2S storage albumin, measured by RAST, was detected in most (96%) castor bean-sensitive patients, confirming it as the major allergen. We would like to suggest that the 2S albumin be named Ric c I, that the crystalloid proteins be named Ric c II, and that the 47/51 kd doublet be named allergen 3.