Soluble or particulate microbial antigens are excreted in the urine in many systemic infectious processes. The ease with which urine antigens can be concentrated has facilitated their detection by immunologic methods. The group and type-specific bacterial polysaccharides are among the best studied examples of urinary excretion of microbial antigens. These polysaccharides are often present in the urine as low molecular weight fragments (70,000 daltons or less) and in some instances may represent degradation products of the native polysaccharides. Urine polysaccharides are sufficiently immunoreactive to be detectable by simple precipitin and agglutination techniques in a large percentage of patients with infections due to certain pyogenic bacteria including Haemophilus influenzae and group B streptococci. Both polysaccharide and protein antigens have been detected in the urine by immunologic methods in numerous other infections including parasitic, viral, and spirochetal diseases. Detection of a thermostable antigen in the urine of patients with Legionnaires' disease by radio- and enzyme-linked immunoassays represents an important recent advance. The exact role of immunologic tests for etiologic diagnosis in infectious diseases is not established, but will undoubtedly be influenced by developments such as monoclonal antibody technology and better availability of standardized immunologic reagents.