Immunophenotyping is widely used to aid in understanding changes to the immune system by identifying shifts in subpopulations of blood cells. The primary method used is flow cytometry. Immunophenotyping can be used with body fluids such as blood and cerebral spinal fluid and also with solid tissues which can be suspended as single-cell suspensions. Immunophenotyping in nonclinical studies conducted to support drug development and approval is commonly used in regulatory submissions, primarily for immunomodulatory agents, to add to the understanding of the agent’s effect on the immune system. The impact of changes in lymphocyte subsets and potential immunotoxicity is evaluated in a similar manner to other forms of toxicity (e.g., readily reversible effects with a large safety margin raise less concern than slowly reversible or nonreversible effects with a more limited safety margin) and interpreted in the context of the overall nonclinical study.