I discuss the relationship between theoretical terms and measuring devices using a very peculiar example from biomedical research: cancer transplantation models. I do so through two complementary comparisons. I first show how a historical case study can shed light on a similar case from contemporary biomedical research. But I also compare both to a paradigmatic case of measurement in the physical sciences – thermometry – which reveals some of the most relevant epistemological issues. The comparison offers arguments for the recent debate on the operational definition of Cancer Stem Cells, and thereby suggests the relevance of a comparative approach in the history and philosophy of science.