Compositions suitable for use as signal generation components of an immunoassay, and methods for their use. According to one aspect of the invention, the composition includes a carrier having a coating of an aminodextran and a metal chelate incorporated therein. The metal chelate is present in the amount of at least 0.065 .mu.Mole per gram of carrier, and the aminodextran coating density averaging at least about 45 .mu.g per milligram of carrier. In another aspect of the invention, carrier is dyed with a complex having the formula: M(L1).sub.x(L2).sub.y, wherein M is a metal selected from the group consisting of europium, terbium, dysprosium, samarium, osmium and ruthenium; L1 is a ligand selected from the group consisting of DPP, TOPO, TPPO; L2 comprises a ligand having the formula ##STR00001## wherein R is one or more substituents, each substituent comprising an electron donating group; n=2-10; x=1-2; and y=2-4.