Enzyme activators lower K[subscript m] (the Michaelis constant) and/or raise V[subscript max] (the asymptotic reaction velocity at infinite substrate concentration); conversely, inhibitors raise K[Subscript m] and/or lower V[subscript max]. But what if an effector moves both K[subscript m] and V[subscript max] in the same direction? Uncompetitive inhibitors, which decrease both K[subscript m] and V[subscript max] by the same factor, are the most common example of this. A less well-known example occurs often when crowding agents are added to the buffer in order to mimic the environment commonly encountered "in vivo". Crowding agents tested on different enzymes have been shown to have every conceivable effect on K[subscript m] or V[subscript max], causing them to rise, fall, or stay the same. In this article, a mathematical analysis is presented allowing biochemists to judge whether an effector that causes K[subscript m] and V[subscript max] to both move in the same direction serves as an inhibitor, an activator, or most surprising, as both.