Cytotoxicity of 12 3,4-dihydroisocoumarins was analyzed in the sensitive lung cancer cell line A549 as well as in three resistant cell lines HCT-15 (colon carcinoma), MCF-7/MX (breast carcinoma) and H69AR (lung carcinoma) either expressing the ABC transporter P-gp, BCRP, or MRP1, respectively. Here, ABCG2 is linked to breast carcinoma.