The activity of multiple proteins in a single living cell, portion of a cell or in a group of cells is simultaneously measured by introducing reporter molecules into the cell(s) or a portion thereof, chemically modifying the reporter(s) by the enzyme of interest, terminating the modification reactions, removing the reporter(s) and modified reporter(s), and determining the amount of enzyme activity present by measuring or comparing the amount of reporter(s) and modified reporter(s) present. By performing a series of experiments at different time points, conditions, and varieties of cell types, a database is developed for molecular cellular mechanisms in health and disease states. By exposing cells to a variety of compounds data for drug development and screening is provided.