'Lysogeny is the hereditary power to produce bacteriophage'. This definition, coined by André Lwoff in 1953, seems simple enough. However, it summarizes a very complex history, which began with the discovery of bacteriophages. How was the novel relationship between a virus and a bacterial cell conceived? In what way did this relationship renew the question of the nature of viruses? How did it generate a theory of hereditary factors? It was soon shown that bacteria can produce a lysogenic agent without (...) exogenic phages. That was in 1925 with the discovery of lysogeny. In an attempt to throw light on the history of lysogeny, I have traced the work of Eugène Wollman from 1919 to 1943, and have examined the work of J. Bordet, O. Bail and F.M. Burnet. The second part of this paper is devoted to the experiments that led to the elucidation of lysogeny. Of particular importance was the shaping of the concept of 'prophage'. Lysogeny was studied anew and clarified at the Institut Pasteur in Paris under the guidance of André Lwoff. How was genetic determinism of lysogeny brought to light? What new models and new concepts then formed the basis of genetic analysis of the relationship between the virus and its bacterial host? These aspects are seen in the light of the work of André Lwoff, François Jacob, Joshua Lederberg and Elie Wollman. (shrink)

SummaryIn the 1940s a controversy developed between John H. Northrop (Nobel Laureate, 1946) and Max Delbrück (Nobel Laureate, 1969) on the formation of bacteriophage. From the historiography of molecular biology there emerges a picture of an obstinate Northrop who repudiated the ‘correct’ insights revealed by the experiments of Delbrück. The established reputation of Northrop confronts one with the question of why Delbrück's epoch-making experiments were not convincing for Northrop. It will be argued that this was a consequence of local incommensurability (...) between Northrop's physiological/chemical research style and Delbrück's bacteriological/genetic research style. The analysis of this controversy reveals the elements that made up these two research styles. (shrink)