Marine sponges have been shown to produce metabolites with cell growth- and endocrine-altering activities. We tested extracts from two species: the ‘brown variable sponge’ (Anthosigmella varians) and the ‘West Indian bath sponge’ (Spongia barbara), for effects on the cell cycle regulatory protein, cyclin B1; cell cycle growth-phase (sub-G1/apoptosis, G1, S, and G2/M); and cell survival in SW-13 human adrenal carcinoma cultures. Polyacrylamide gel electrophoresis studies indicated a 70–90% reduction in cyclin B1 levels by treatment with these agents. Microscopic examination of cultures with DAPI staining showed dense and fragmented DNA fluorescence, characteristic of apoptosis, in both sponge extract-treated cultures but not in controls. Flow cytometry analysis showed a 16-fold increase in the percentage of cells entering apoptosis (sub-G1 phase of cell cycle) by treatment with Anthosigmella varians extract (p<0.01) and a 10-fold increase using Spongia barbara extract (p<0.01). During this same time, the percentage of cells in G2/M was increased 1.6-fold by Anthosigmella varians extract (p<0.01) and 2.0-fold by Spongia barbara extract (p<0.01). Cell growth/survival studies also indicated a time-dependent decline in the percentage confluence of cell cultures exposed to Anthosigmella varians or Spongia barbara extracts. These experiments demonstrate that some species of marine sponges have metabolites which are capable of interfering with the mammalian cell cycle and with the survival of human adrenal carcinoma cells in culture.