Experiments were carried out in a specially designed 0.025 m i.d., 5 m high gas upflow-solids downflow fluidized bed to measure axial pressure gradient profiles and actual solids holdups. Fluidized catalytic cracking (FCC) particles (rho (p) = 1300 kg/m(3), d(p) = 70 mum) and two types of glass beads (rho (p) = 2500 kg/m(3), d(p) = 123 and 332 mum) were used. The results show that such counterflow is possible over a large range of operating conditions. The flow development and friction are discussed in relation to the pressure gradient profiles. Based on the changes of the mean particle velocity and the particle slip velocity, the particle agglomeration was studied. The actual solids holdup measured by a pair of pinch valves and the apparent solids holdup calculated from the pressure gradients are also compared for different operating conditions.