The protein target of the ammosamides, cytotoxic natural products from a marine-derived actinomycete, has been elucidated. An immunoaffinity fluorescent tag was used to construct a fluorescent molecular probe (see structure). First, uptake and localization of the probe in cells was visualized with fluorescence microscopy. The probe was then used to co-immunoprecipitate proteins that bound to the ammosamide core. Myosin was thus identified as a target.