The Guava Cell Cycle Assay allows quick determination of G0/G1, S and G2/M cell cycle phases. Cell cycle phase percentages and counts are displayed on the same screen as acquisition, without requiring software set-up, making it easy to visualize the results in both histogram and statistical formats. Cell counts for each sample are direct and absolute, without the use of reference beads. This allows researchers to monitor whether changes in cell cycle are a result of treatments and not just changes in cell number due to cell proliferation or cell death. The optimized reagent requires no sample preparation and includes detailed protocols for adherent and suspension cells in both 96-well plates and tubes to reduce assay development time. Understanding the cell cycle and how different compounds affect it is an important area of study for immunology and cancer researchers, as well as in drug discovery and process development.