Zoo-born mammals can contribute to conservation translocation programmes; the real questions are when and how they can contribute, and how often they should or actually do contribute. The “when” is definitely when a species is extinct in the wild and, by chance or by design, is held in zoos, the European bison being a classic, but rare, example. In other situations zoo-born mammals or their offspring could contribute a proportion of appropriate release stock, that proportion being larger or smaller depending on numerous factors. Delving into the details of those factors is the “how” part of the question, including aspects such as taxonomy, genetics, disease risk, behavioural and welfare considerations, and impacts on the viability of the source or the destination population. Personal experience as a conservation translocation practitioner suggests that the cumulative effect of these factors leads to an often strong negative view of the potential of zoo-born mammals to contribute to conservation translocations. Other researchers have tried to quantify the “how often do they contribute” question, finding that captive-born mammals are rarely used in conservation translocations, and even when they are, zoos are usually not the source of the captive-born individuals. I will draw on these meta-analyses and on personal experience of conservation translocations with large mammals such as gorillas, gibbons, rhinos and cheetahs to explore the “how often should zoo mammals contribute” question, and reflect on whether the significance of these contributions is sufficient to justify maintaining zoo populations for this purpose.