Provided herein are methods that enable parallel evaluation of multiple functional nucleic acids in individual cells or subpopulations of cells, in the context of incubation with other types of single cells. The key insight is concurrent measurement of polynucleic acids derived from small populations of at least two different cell types, such that function in one cell type is linked to the clonal identity of another cell. These methods simultaneously process thousands, millions, or more single cells or small populations of cells. The method integrates molecular, algorithmic, and engineering approaches. This invention has broad and useful application in a number of biological and medical fields, including immunology and drug discovery.