Method for testing an in-riser landing string assembly (150; 151,152) onboard a floating vessel, which in-riser landing string assembly is configured to, in order to conduct one or more operations, be deployed by means of landing string tubing from said vessel inside a marine riser, which riser extends between said vessel and subsea well equipment (205,206), wherein an umbilical (161;161a,b) extends through the marine riser down to the deployed in-riser landing string assembly, wherein the method comprises, onboard said vessel, testing the in-riser landing string assembly connected to the umbilical prior to deployment of the in-riser landing string assembly, said deployment taking place at a deployment location where the in-riser landing string assembly is above the marine riser, characterized in that, the method comprises the use of an in-riser landing string assembly cart (130;230) having a cart base (131;231) and having a support tower (140;240) erected on said cart base, wherein the in-riser landing string assembly is arranged and retained in vertical orientation on said cart (130;230) with the support tower (140;240) providing lateral support for the in-riser landing string assembly, and wherein the cart (130;230) with said in-riser landing string assembly (150;151,152) arranged and retained in vertical orientation thereon is arranged at a testing location that is remote from the deployment location, wherein one or more tests are performed on the in-riser landing string assembly (150;151,152) and/or components thereof whilst at said testing location, with the umbilical (161;161a,b) connected during said one or more tests, and wherein the cart with said in-riser landing string assembly arranged and retained in vertical orientation thereon is then moved from said remote testing location to said deployment location above the marine riser. wherein the method comprises, onboard said vessel, testing the in-riser landing string assembly connected to the umbilical prior to deployment of the in-riser landing string assembly, said deployment taking place at a deployment location where the in-riser landing string assembly is above the marine riser, characterized in that, the method comprises the use of an in-riser landing string assembly cart (130;230) having a cart base (131;231) and having a support tower (140;240) erected on said cart base, wherein the in-riser landing string assembly is arranged and retained in vertical orientation on said cart (130;230) with the support tower (140;240) providing lateral support for the in-riser landing string assembly, and wherein the cart (130;230) with said in-riser landing string assembly (150;151,152) arranged and retained in vertical orientation thereon is arranged at a testing location that is remote from the deployment location, wherein one or more tests are performed on the in-riser landing string assembly (150;151,152) and/or components thereof whilst at said testing location, with the umbilical (161;161a,b) connected during said one or more tests, and wherein the cart with said in-riser landing string assembly arranged and retained in vertical orientation thereon is then moved from said remote testing location to said deployment location above the marine riser.