New anticancer drugs are needed to support cancer treatment. Cancer involves the uncontrolled proliferation of tumor cells, and many anticancer drugs are agents that inhibit tumor cell growth. The antitumor cell N-myristoylated-peptide is a natural product purified from Heliothis virescens insect hemolymph with essentially no cytotoxicity against human foreskin fibroblast cells. A synthesized structure of six amino acids (Myristoyl-Cys-Ala-Val-Ala-Tyr-[3 methyl]His-OMe; M.W. 902.2Da) was screened at 10μM for in vitro antitumor activity against 56 human tumor cell lines by the National Cancer Institute (NCI). The NCI found that 34 of the 56 tumor cell lines (representing eight major tumor types) were sensitive (>50% growth inhibition of tumor cells) to the myristoylated-peptide. The best overall antitumor activities (>60% average growth inhibition) were seen against 17 tumor cell lines for non-small cell lung cancer, leukemia and melanoma and for eight other tumor cell lines. The NCI finds that their in vitro screen is an effective selector of compounds with in vivo antitumor activity.