Advocates of ''Mendelism'' early on stressed the usefulness of Mendelian principles for breeders. Ever since, that usefulness-and the favourable opinion of Mendelism it supposedly engendered among breed- ers-has featured in explanations of the rapid rise of Mendelian genetics. An important counter-tradition of commentary, however, has emphasized the ways in which early Mendelian theory in fact fell short of breeders' needs. Attention to intellectual property, narrowly and broadly construed, makes possible an approach that takes both the tradition and the counter-tradition seriously, by enabling a more complete description of the theory-reality shortfall and a better understanding of how changing practices, on and off the Mendelians' experimental farms, functioned to render that shortfall unproblematic. In the case of plant breeding in Britain, a perennial source of lost profits and disputes over ownership was the appear- ance of individual plants departing from their varietal types-so-called ''rogues.'' Mendelian plant varie- ties acquired a reputation for being rogue-free, and so for demonstrating the correctness of Mendelian principles (and the genius of Gregor Mendel), at a time when Mendelians were gradually taking control of the means for distributing their varieties. Mendelian breeders protected their products physically from rogue-in ducing contamination in such a way that when rogues did appear, the default explanation-that ???contami nation had somehow occurred-ensured that there was no threat to Mendelian principles.