The absolute configuration of the cytotoxic guanidine alkaloid monanchocidin A with 11 stereogenic centers from the marine sponge Monanchora pulchra was determined as 5 R, 8 S, 10 S, 13 R, 14 S, 15 R, 19 R, 23 R, 37 S, 42 S, 43 R after extensive reductive degradation and conversion of the resulting alcohols to MTPA derivatives.