Lichens are resilient organisms, known for their unique profiles of secondary metabolites and for exhibiting antioxidative, antibacterial, and cytotoxic effects. Analyzing the cytotoxic potential of Lobaria scrobiculata, a bioassay-guided fractionation strategy yielded seven known metabolites, with two of these compounds, 2 and 3, exhibiting cytotoxicity against HL-60 cells. In order to verify the potential impact of degradation on observed bioactivity, a purity and stability evaluation was conducted. The consistency of results obtained by the water-soluble tetrazolium salt-1 assay and trypan blue cytotoxicity assay was evaluated for selected compounds.