Cytarabine is one of the standard drugs used in treating acute myeloid leukemia. It is noted that cytarabine causes cell cycle arrest in leukemia cells in different phases of the cell cycle. Arrest in the S and G2/M phases activates Chk1 which in turn inhibits CDK1, but the role of CDK1 in the effects of cytarabine has not yet been described. The aim of this paper is to test the effect of cytarabine on the cell cycle of U937 cells and kinase CDK1, while the hypothesis is that cytarabine will halt progression of U937 cells through the cell cycle and inhibit CDK1. I marked the cells treated with cytarabine with propidium iodine and determined the percentage of cells in each phase of the cell cycle using flow cytometry. To determine the level of CDK1 phoshorylated on Tyr-15 in cells treated with cytarabine for 48 hours I used Western blot analysis. The results show that cytarabine lowers the number of living cells in a dose dependent manner and in higher concentrations causes cell cycle arrest in the S and G2/M phase. Cell cycle arrest correlates with the phoshorylation of CDK1 on Tyr-15 which indicates the role of CDK1 as a key molecule in the ATR-Chk1 signaling pathway in the effects of cytarabine on U937 leukemia cells.