A method for handling and/or testing an in-riser landing string assembly onboard a floating vessel, which method comprises the use of an in-riser landing string assembly cart, e.g. a skiddable in-riser landing string assembly cart, having a cart base, e.g. a cart base configured to be skidded over skid rails on a deck of the vessel, and having a support tower erected on said cart base. The in-riser landing string assembly is arranged and retained in vertical orientation on said cart with the support tower providing lateral support for the in-riser landing string assembly, e.g. the in-riser landing string assembly being temporarily secured to the tower at different elevations along the height of the support tower.