Economists and legal scholars have debated the reasons people adopt opensource software, and accordingly whether and to what extent the open-source model can scale, replacing proprietary rights as a primary means of production. In this Article, we use the release by a biotechnology company of similar software under both proprietary and open-source licenses to investigate who uses open-source software and why. We find that academic users are somewhat more likely to adopt open-source software than private firms. We find only modest differences in the willingness of open-source users to modify or improve existing programs. And we find that users of open-source software often make business decisions that seem indifferent to the norms of opensource distribution. Our findings cast some doubt on the penetration of the open-source ethos beyond traditional software markets.