We study the link between a laboratory's personnel composition, its number and types of graduate students and postdocs, and the laboratory's productive output. Building upon a fine-grained dataset with full personnel lists from the MIT Department of Biology from 1966–2000, we find that while postdocs account for the large majority of publication outputs, graduate students and postdocs with external funding contribute equally to breakthrough publications. Moreover, technicians are key contributors to breakthrough publications, but not to overall productivity. Taken together, this study contributes to our understanding of knowledge work, as well as reinforcing the importance of a laboratory's personnel composition.