For testing problem sera, the fluorescent treponemal antibody-absorption test (FTA-ABS) is increasingly replacing other treponemal tests for syphilis (including the TPI test) in the USA. The authors review its development as a result of experience with other immunofluorescent techniques and discuss its advantages and disadvantages. Although it is a more sensitive indicator of past or present syphilitic infection than many other widely evaluated techniques, several aspects of the test are still not fully understood; in particular its specificity is not yet established and more information is needed on its reactivity in a variety of clinical diseases and in treponemal diseases other than syphilis. It is a relatively expensive technique in terms of technicians' time, reagents and equipment but there are grounds for hope that research now under way into automation of the procedure may improve the situation.