Abstract: An automated analytical instrument for conducting assays for components of interest in fluid samples. The instrument includes a testing system for assaying a fluid sample for a component of interest, a fluid dispensing assembly, an assembly for providing an assay module to the testing system and a fluid sample holding tray transport assembly for transporting a fluid sample holding tray carrying fluid sample holding containers to the fluid dispensing assembly and away from the fluid dispensing assembly. The transport assembly includes a conveyor, a fluid sample holding tray loading unit and a fluid sample holding tray unloading unit. The conveyor includes a carriage which is slidably mounted on a rail and moved along the rail by an endless belt which is fixedly attached to the carriage and driven by a reversible motor. In the operation of the instrument, one or more fluid sample holding trays are placed onto the fluid sample holding tray loading unit.