The invention provides a novel class of reactive fluorescent agents that are based on a pyrene sulfonic acid nucleus. The agents are readily incorporated into conjugates with other species by reacting the reactive group with a group of complementary reactivity on the other species of the conjugate. Also provided are methods of using the compounds of the invention to detect and/or quantify an analyte in a sample. In an exemplary embodiment, the invention provides multi-color assays incorporating the compounds of the invention.