According to string-five-brane duality, the Green-Schwarz factorization of the D = 10 spacetime anomaly polynomial I12 into X4X8 means that just as X4 is the anomaly polynomial of the d = 2 string world sheet, so X8 should be the anomaly polynomial of the d = 6 five-brane world volume. To test this idea we perform a five-brane calculation of X8 and find perfect agreement with the string one-loop result.