Grating-coupled surface plasmon resonance (GCSPR) is a method for the accurate assessment of analyte in a multiplexed format using small amounts of sample. In GCSPR, the analyte is flowed across specific receptors (e.g. antibodies or other proteins) that have been immobilized on a sensor chip. The chip surface is illuminated with p-polarized light that couples to the gold surface's electrons to form a surface plasmon. At a specific angle of incidence, the GCSPR angle, the maximum amount of coupling occurs, thus reducing the intensity of reflected light. Shifts in the GCSPR angle can be correlated with refractive index increases following analyte capture by chip-bound receptors. Because regions of the chip can be independently analyzed, this system can assess 400 interactions between analyte and receptor on a single chip. We have used this label-free system to assess a number of molecules of immunological interest. GCSPR can simultaneously detect an array of cytokines and other proteins using the same chip. Moreover, GCSPR is also compatible with assessments of antigen expression by intact cells, detecting cellular apoptosis and identifying T cells and B cells. This technology represents a powerful new approach to the analysis of cells and molecular constituents of biological samples.