Three new cytotoxic compounds, one a cyclic-peroxide-containing acid (5) and the two others alkylated dihydroxy alpha,beta-unsaturated C22 and C21 acids (6 and 7), were isolated from the marine sponge Plakortis halichondrioides, which was collected off the coast of Jamaica. The structures were elucidated through mass and mainly 1D and 2D nmr spectral analysis. All three acids are cytotoxic against P-388 murine leukemia.