Methods, compositions and kits are disclosed. The compositions are light
emitting and comprise a polymeric matrix having dissolved therein a
photoactive compound. The composition has the characteristic that, after
activation of the photoactive compound, the rate of decrease in the
intensity of light emission at any time during a 20-fold decrease in the
intensity is proportional to the intensity of the light emission. In one
embodiment the polymeric matrix is comprised of particles of about 20 nm
to about 100 .mu.m in diameter to which is bound a specific binding pair
member. The particles generally comprise a polymeric matrix having
dissolved therein about 1 to about 20% by weight of a dopant. The
compositions may be used in methods for determining an analyte. A
combination is provided comprising (1) a medium suspected of containing
the analyte, (2) and the aforementioned composition. The photoactive
substance is activated and the effect of the activating on the optical
properties of the combination is detected. The presence and amount of the
effect is related to the presence and amount of the analyte in the
medium. Also disclosed are kits for use in an assay.