A cell-based (in-cell) ELISA measures a protein of interest directly in fixed cells in a 96-well plate — no cell lysis, protein extraction or gels required. Cells are cultured, treated, fixed and immunostained in situ, and a target-specific primary antibody is detected with an enzyme-conjugated secondary. The colorimetric (or fluorometric) signal is proportional to the amount of target protein and is normalised to cell number in each well.

This lets you quantify total and phosphorylated proteins across many samples at once — a faster, higher-throughput alternative to western blotting for screening the effects of activators and inhibitors on signalling.