The immiscible displacement of a viscous Newtonian liquid by a second Newtonian liquid or a viscoplastic material in a capillary tube is experimentally analysed in the low inertia regime and very similar mass densities. The dimensionless numbers that govern the problem are the capillary number (Ca), the viscosity ratio of the displaced to the displacing fluids (N-eta), and the plastic number, PI, a dimensionless normalized yield stress. The tube is initially filled with a fluid when a second fluid is injected with a flow rate control, displacing the first one. The displacement efficiency of the Newtonian Newtonian case is found to be in good agreement with the numerical data of the literature. Pictures of the flow reveal that for N-eta < 2 the radius of the displacing fluid varies along its length, being thinner near the front of the interface. In addition, within this range, buoyant effects can arise even for low discrepancies in mass density. We found that the displacement efficiency and geometric mass fraction decay with the increase of plastic effects if we define the viscosity ratio taking into account the yield stress in the viscoplastic material. We also found that plastic effects associated to the injected material can be responsible for the appearance of irregular (non-parallel) interface with respect to the wall. (C) 2017 Elsevier B.V. All rights reserved.