Abstract Despite progress in immunotherapies, there are significant challenges to overcome to make them more broadly applicable to different cancers and more effective for patients. However, current efforts to improve adoptive cell therapies and immune checkpoint blockade as well as cancer vaccines are hampered by the lack of a method to characterize the functional and phenotypical changes of different subpopulations of T cells over time at high throughput. The goal of this SBIR Phase II project is to develop a novel flow-cytometry method that can solve this technological bottleneck. The time-lapse flow cytometry uses laser particle (LP) barcodes to track each cell across flow measurements taken at different time points. Built on demonstrated proof-of-concept data, this project is focused on establishing a set of novel assays for characterizing T cells in response to different stimuli in a time-resolved manner. The first specific aim is to demonstrate short-term time-lapse assays to profile cytokine secretion (t-SEC). Secretion of TNF, IFNγ and IL10 from millions of the same cells in response to different types of stimulation are measured at multiple timepoints over 24 hours. The second specific aim is to demonstrate long-term time-lapse assay to monitor phenotype (t-PHENO), including T-cell exhaustion over 10 days. The third specific aim is to validate the t-SEC and t-PHENO assays with patient samples. The time- resolved, high-throughput, high-parameter assays are expected to accelerate the development of more effective and durable immunotherapies for cancer (>$100 B global market). Beyond immunotherapy, time-lapse flow cytometry is also expected to be useful in fundamental immunological research and vaccine development.