Dynamin is an important GTPase that has been implicated in membrane fission during clathrin-mediated endocytosis. In mammals, three dynamin isoforms can be found, with dynamin 1 and 3 mainly expressed in the brain, and dynamin 2 being ubiquitous. In accordance with a crucial role in endocytosis, knockout (KO) of dynamin 2 in mice results in embryonic lethality. To assess the impact of the lack of dynamin on cell physiology, Pietro De Camilli and colleagues had previously generated dynamin 1 and 2 double KO (DKO) mouse fibroblasts, which – unexpectedly – were viable despite a severe impairment of clathrin-mediated endocytosis. To exclude that low levels of dynamin 3 might have compensated for the loss of dynamin 1 and 2, in this work (p. 5305), the authors generate triple KO (TKO) mouse fibroblast cells that lack all dynamin isoforms. They show here that these dynamin TKO cells have the same phenotype as the DKO cells without any further defects. The authors then proceed to use the dynamin TKO cells to investigate the specificity of the commonly used small molecule dynamin inhibitor dynasore and of its structurally related compound Dyngo-4a. Surprisingly, they find that the dynamin TKO cells are sensitive to dynasore and Dyngo-4a; treatment with these drugs blocked fluid-phase endocytosis and peripheral membrane ruffling, both in TKO and wild-type cells. On the basis of these results, the authors propose that the dynamin TKO cells are a useful tool to unambiguously assess the cellular functions of dynamin and to test the specificity of dynamin inhibitors.