Single-cell multiplexed proteomics has provided an unprecedented insight into the functional heterogeneity of phenotypically similar T cells. We employ a single-cell, 17-plex cytokine assay microdevice and new data visualization methods to fully evaluate the functional profile of chimeric antigen receptor (CAR)-modified T cells targeting CD19, which have shown promising clinical efficacy in patients with B cell malignancies. The CAR-T cells were manufactured from human peripheral blood mononuclear cells (PBMCs) transfected with lentivirus encoding the CD19-BB-z transgene. CD4+ and CD8+ CAR-T cells were stimulated with anti-CAR microbeads or control IgG at 37°C, 5% CO2 for 24 hours. After stimulation, cells were loaded into a single-cell barcode chip (SCBC) containing ~12000 microchambers, each pre-patterned with a complete copy of a 17-plex antibody array and further incubated for 16 hours. We demonstrate the marked heterogeneity and polyfunctionality (2+ secreted cytokines) of individual CD19 CAR-T cells across 4 donors, comprising anti-tumor effector profiles mixed with a range of stimulatory, immunosuppressive, and immunotoxic/proinflammatory functions. In order to take full advantage of the high dimensionality of this data, we present new bioinformatics visualizations and dimensionality reduction techniques for effective combinatorial polyfunctionality analysis. Our analysis reveals a complex yet meaningful landscape of immune effector response in CD19 CAR-T cells to antigen-specific challenge. The ability to dissect polyfunctional subsets has the potential to guide CAR-T product pre-infusion quality assessment and result in CAR-T cells engineered with more efficacious and less immunotoxic profiles.