Test kit containing labeled receptor and wash solution for determination of an immunological ligand

A test kit useful for the determination of an immunological ligand includes a labeled receptor for that ligand, a specific wash solution and a test device. The wash solution is buffered to a pH of from about 5 to about 9 and contains at least 0.01 weight percent of a compound having a dodecyl sulfate anion and an alkali metal or ammonium cation. One such compound is sodium dodecyl sulfate. The test device has several test wells, and in each well there is a microporous membrane. A receptor for the ligand of interest is immobilized in one test well. The kit is particularly useful for measuring human chorionic gonadotropin as an early indicator of pregnancy.