A novel nitrogenous macrolide, designated salarin C (3), was isolated from the Madagascan sponge Fascaplysinopsis sp. The structure of the compound was elucidated by interpretation of MS and 1D and 2D NMR spectra. Salarin C is closely related to salarin A and is considered to be the precursor of salarins A and B (1,2). Air oxidation was found to transform 3 to 1. Salarin C was found to inhibit cell proliferation of human leukeamic cell lines UT-7 and K562 and the murine pro-B cell line Ba/F3 at concentrations of 0.0005-0.5 mg/ml. A possible biogenesis is discussed.