Analytical reagent reaction vessel and method for performing sequential analytical assay procedures to determine an analyte in a liquid test sample. The reaction vessel is in the form of a closed container having a substantially horizontal axis of rotation and incorporated with one or n:ore analytical reagents for performing a desired analytical assay procedure. The reaction vessel is designed to provide free gravitational movement of a liquid test sample disposed therein. The analytical reagents are incorporated into the reaction vessel such that they are contacted with a liquid test sample in a desired order or sequence. A liquid test sample disposed in the reaction vessel is capable of being manipulated therein by rotating the reaction vessel about the horizontal axis wherein the liquid test sample is transported by gravity. Accordingly, a liquid test sample can be sequentially contacted with the analytical reagents, and one or more of a detectable response can be measured, without the need for additional external manipulative steps or complicated instruments to complete the assay. The reaction vessel is particularly useful for performing immunoturbidimetric and immunometric assays.