Technical Abstract: We recently demonstrated that treatment of three leukemic cell lines with an aqueous extract of cinnamon (CE) for 24 h produced dose-dependent arrests in the G2/M phase of the cell cycle. To accomplish the goal of understanding underlying mechanisms, we selected the cell line most responsive to the CE treatment to study the effects of the extract on signaling molecules regulating cell cycle progression. Cell cycle analyses were conducted on treated (CE =0.1mg/mL) versus non-treated cells from 0 - 6 h. The percentages of cells in G2/M in CE-treated cells increased significantly from 11.0 +/- 1.0 to 23.6 +/- 1.4 after 6 h, while the percentage for non-treated cells remained unchanged (12.3 +/- 0.8). Multiparametric flow cytometric analyses were used to associate activation of p38 MAPK with cells arrested in G2/M, the size of these cells, and the presence or absence of cyclin B1. After 4 h, there was a 26% increase in the activated phosphorylated form of p38 MAPK in CE-treated cells compared to the non-treated control cells, with larger cells showing the greater increase. Although the proportion of CE-treated cells in G2/M was higher than controls, this population was shown to be less positive for cyclin B1 than the control G2/M population. Our results demonstrate that CE significantly modulated two signaling proteins, p38 MAPK and cyclin B, that regulate progression through G2/M. Overall, the data provide evidence that CE affects tumor cell proliferation by disrupting critical phosphorylating/dephosphorylating signaling events that propel cells through the G2/M phase.