A red-cell-linked antigen-radioactive antiglobulin reaction (RCLA*AR) is reported which may be combined with the conventional red-cell-linked antigen-antiglobulin reaction (RCLAAR) to measure antibodies of all five immunoglobulin classes against the same determinant at the same time. The method has been applied to sera from penicillin-allergic patients and controls. Human erythrocytes coated with rabbit antibodies carrying penicilloyl groups were incubated with the serum to be tested, then washed. ¹²⁵I-labelled anti-human IgE was added and, after further incubation and washing, the radioactivity retained by the cells was counted. This gives a measure of the specific IgE in the test sample. Other aliquots of the coated cells were treated with specific antiglobulin reagents in the usual manner of the RCLAAR in order to determine antibodies of other classes. The sensitivity for IgE was similar to that of the radioallergosorbent test. Information was also obtained about specific IgG subclasses.