The Micronesian marine sponge Phakellia sp. has been found to contain cern key members of the antineoplastic halichondrin/halistatin family. These compounds include halichondrin B, homohalichondrin B, halistatin 1, and halistatin 3.Halistatin 3 is a newly discovered member of this family. It inhibited both the P388 leukemia cell line and selected brain, lung, colon, ovarian, renal, and melanoma type cancer cell lines with ED.sub.50 /GI.sub.50 concentrations on the order of 3.times.10.sup.-5 .mu.g/ml.