Yes, you can take a 3.4 oz (100ml) deodorant on a plane, but the rules for how you pack it depend on its physical state. If your deodorant is a liquid, gel, cream, or aerosol spray, it must follow the TSA "3-1-1" rule: it must be in a container of 3.4 ounces or less and fit inside a single, quart-sized, clear plastic zip-top bag. A standard 3.4 oz container is the absolute maximum size allowed for these types in a carry-on. However, if your deodorant is a solid stick, it is not considered a liquid and does not have to comply with the 3.4 oz limit; you can bring a large, full-sized solid deodorant stick in your carry-on without placing it in your liquids bag. This distinction is important because many "gel-stick" deodorants are classified as gels by security and will be confiscated if they are larger than the limit. To ensure a smooth security experience, it is always best to check the label; as long as the container itself is marked 3.4 oz/100ml or smaller, it is legal to bring regardless of whether it is a liquid or solid.