Antiproliferative bioassay-guided fractionation of an aqueous extract of the marine sponge Chondropsis sp. provided two novel macrolides, chondropsins A (1) and B (2). The structures were elucidated by a combination of spectroscopic analysis and chemical modification. HMBC, TOCSY, NOESY, and HSQC-TOCSY experiments were particularly useful for the structural assignments of these polyketide-derived metabolites. The chondropsins define an unprecedented class of polyunsaturated, polyhydroxylated, 35-membered macrocycles which incorporate both lactone and lactam functionalities. An additional unique feature of these compounds is a complex, amide-linked, polyketide side chain. Testing of chondropsin A (1) in the NCI 60-cell screen revealed a mean-graph profile that did not correlate significantly with the profile of any compound class represented in the NCI standard agents database. The chondropsins therefore represent an interesting new lead for cancer therapeutics research.