The capacity of preparing serums and vaccines that were able to cure or to immunize people was presented by the bacteriologists of the end of the 19th century as one of the most important achievements of the « microbiological revolution ». However, the difficulties encountered during the development of a therapeutic product elongated the path between the conception of such a product and its standardization and utilization in large scale. This discrepancy between speech and reality can be illustrated by the analysis of the trajectory of the antiplague serum, conceived between 1894 and 1895 at the Pasteur Institute. The greatest difficulty that slowed down its stabilization was the fact that it was tested in India, and not in France, during the epidemics of the bubonic plague of 1897 and 1898. The analysis of letters and notebooks belonging to the two researchers responsible for the essays, Alexandre Yersin and Paul Simond, reveals their strategies and the relationships that they had to develop with the local actors to achieve their goals. It also unravels the inevitable competition and controversies opposing Yersin and Simond to other European researchers who were in India to test their own products against the plague.