The problem of whether it is theoretically possible to determine antigens and antibodies in a cross-reacting serological system is analyzed. Conditions are listed which will insure that the serological operations of absorption, elution, blocking, and counterblocking uniquely determine antigens and antibodies. There is no restriction on cross-reactivity. The protocol for counterblocking was given by Denniston and has not been developed by serologists. Denniston has shown that counterblocking can be replaced by combinations ofthe other operations in a system where there can be no cross-reactivity. This paper indicates the potential usefulness of such a technique.