Ludwik Fleck is widely recognized as a precursor of Science and Technology Studies, but his case study on the development of the Wassermann reaction as a test for detecting syphilis has never been subjected to detailed empirical scrutiny. The fact that Fleck?s monograph is based on a limited set of documentary sources makes his work vulnerable to uncharitable critics. The problematic relation between thought collective and individual scientists in Fleck?s theoretical approach is another reason for a systematic re-examination of his case study, using materials on the early period in the history of the Wassermann reaction (1906?1912). My re-examination highlights several problems in Fleck?s account: a misinterpretation of the switch from antigen detection to antibody detection; a neglect of the ?clinical connection?; an overemphasis on the importance of collective experience leading to implausible views on gross retrospective distortions supposedly inflicted by this experience upon the memories of individual participants; and, finally, a misjudgement of the significance of the acrimonious dispute over the intellectual ownership of the Wassermann reaction. What remains unscathed is Fleck?s picture of a zig-zag course of development from false initial assumptions via detours and cul-de-sacs to a clinically usable test in the end.