Claycomb Medium, named after Dr. William Claycomb who established the HL-1 cell line, is specifically designed for the growth of murine cardiomyocytes. HL-1 is the first cell line established that can maintain the differentiated cardiomyocyte phenotype and contractile activity in vitro. The HL-1 cell line can be used for the study of cardiac cell hypertrophy that follows myocardial infarction, the testing of novel cardiac therapeutic drugs and treatments, the production of high levels of cardiac proteins and the study of mature cardiomyocyte specific genes. Claycomb Medium, when supplemented with 100 μM norepinephrin, 10% fetal bovine serum (FBS) and 4 mM L-glutamine, will maintain the HL-1 cell line and the mature cardiomycyte behavior. While observing the HL-1 cells under light microscopy, individual and groups of cells can be observed contracting, becoming more frequent as the cardiomyocytes reach confluency.