Abstract: An ultraviolet light beam is split into two beams which are directed toward a reference cell and a sample cell, respectively. A mechanical rotating light chopper permits the reference and sample cells to be alternately illuminated. Radiation from the reference and sample cells is directed toward and sensed by a single photomultiplier tube. An up/down counter counts the number of photons impinging on the photomultiplier tube from the reference and sample cells. Photodiodes in conjunction with the mechanical light chopper cause the up/down counter to count up when photons are arriving from the sample cell and cause the up/down counter to count down when photons are arriving from the reference cell. A source intensity monitor prohibits further measurements after a preselected amount of incident flux has been sensed. The intensity monitor includes a photodetector circuit which senses a portion of the incident light.

Abstract: The performance of double receptor, specific binding assays is improved by use of a receptor complex having the structureA.sub.BL (BL).sub.n A.sub.1wherein BL is a binding ligand, A.sub.BL is a receptor specific for binding ligand, A.sub.1 is a receptor, BL is covalently bonded to A.sub.1 and A.sub.BL is reversibly bonded to BL. Generally A.sub.BL is absorbed onto an insoluble surface and A.sub.1 is an antibody to the substance being assayed. The complex has particular utility in coated tube and rechargeable radioimmunoassay systems.

Abstract: Improved methods for determining very low concentrations of substances present in fluid samples are provided by employing light emitting tracer compounds and (1) counting the photons emitted therefrom while discriminating against noise, nonspecific light, and quenching effects of the sample, or (2) counting the photons emitted therefrom over a predetermined integrated light flux, or a combination of (1) and (2). Further, novel fluorescently labeled low molecular weight antigens are provided which can be employed in competitive binding techniques in which the above described photon counting methods are useful. A homogeneous competitive binding assay, employing photon emitting tracer materials, which eliminates the need for separating bound from unbound materials is also provided. Finally, a modified enzyme amplification technique is set forth employing enzymes active in the bound phase to provide assay techniques useful for extremely low concentration assays.

Abstract: The performance of double receptor, specific binding assays is improved by use of a receptor complex having the structureA.sub.BL (BL).sub.n A.sub.1wherein BL is a binding ligand, A.sub.BL is a receptor specific for binding ligand, A.sub.1 is a receptor, BL is covalently bonded to A.sub.1 and A.sub.BL is reversibly bonded to BL. Generally A.sub.BL is absorbed onto an insoluble surface and A.sub.1 is an antibody to the substance being assayed. The complex has particular utility in coated tube and rechargeable radioimmunoassay systems.