Continued investigation of cancer-cell growth-inhibitory constituents of the blue marine sponge Cribrochalina sp. has led to discovery of cribrostatins 3 (4a), 4 (5), and 5 (4b) in 10(-5) to 10(-7) % of the wet weight. The structure of cribrostatin 3 (4a) was determined by results of high field (500 MHz) (1)H and (13)C NMR and HRMS interpretations. The same general approach to the structures of cribrostatins 4 (5) and 5 (4b) was completed by X-ray crystal structure determinations. Cribrostatins 3, 4, and 5 provided significant cancer cell line inhibitory activities. Cribrostatins 1 and 2(2) and the newly isolated cribrostatins 3-5 displayed antibacterial and/or antifungal activities.