Presently, the rise of new contaminants in the environment has widened the scope of pharmaceutical analyses as to face the demanding new challenges. An increasing tendency for the interconnection and overlap of research fields, such as ecology and biochemistry, is intensifying the demand for new methodologies to be applied to the survey of drugs in unconventional matrices. Integrated in this group are macrophytes, such as the green macroalgae Ulva lactuca, which are under study as to ascertain their ability as indicators of contamination for many substances. Nonetheless, methodologies for extraction and determination of drugs in such matrices are scarce and new studies on the subject are pressing. A new methodology for the determination of the antibiotic furaltadone in U. lactuca by liquid chromatography-tandem mass spectrometric (LC-MS/MS) procedure was developed, optimized and validated following the guidelines of the EC Decision 2002/657. The calibration curves showed linearity above 0.99 (R(2)). The relative standard deviations obtained for repeatability, expressed as CV, were between 15.3 and 20.5 and for reproducibility 25.3 and 28.2 whereas accuracy was in the interval of 88.9-95.5 (%). The limit of decision (CCα) and the detection capability (CCβ) were respectively 5.57 μg kg(-1) and 10.97 μg kg(-1). The method was successfully applied to experimental samples.