In this paper a new method for concurrent checking of an arbitrarily given combinational multi-output circuit f by use of an 1-out-of-3 code is presented. The circuit f is completed by an additional multi-output circuit g in such a way that the XOR-sums of the corresponding outputs of f or of the inverted outputs of f and g are elements of a 1-out-of-3 code. For efficiently checking the 1-out-of-3 encoded outputs also a new self-checking checker based on a three-rail checker cell is proposed. Index Terms: 1-out-of-3 code, on-line error detection, complementary circuit, totally self-checking 1- out-of-3 code checker.