Immunoassay utilizing formazan-prelabeled reactants

An immunoassay procedure utilizing a formazan-labeled primary antibody to detect the presence of an antigen is disclosed. The formazan-labeled antibody is dissolved in a test fluid suspected to containing the antigen. A dipstick having a first section on which primary antibodies are bonded and a second section free of primary antibodies and antigens and blocked to prevent bonding of primary antibodies and antigens is immersed in the test fluid and removed after a select period of time. A difference in color between the first and second sections indicate the presence of the antigen.