The D-D fusion reactivity from JET plasmas has been optimized by using combined neutral beam injection and radiofrequency heating. Reaction rates of up to 2.9 x 10(16) reactions per second (equivalent to neutron emission rates of 1.45 x 10(16) neutrons per second) have been achieved. The best results were obtained in the double-null magnetic configuration or with plasmas limited by the inner wall. By far the largest fraction of the neutron production originates from beam induced reactions, although, in some cases, about 25% of the production can be attributed to the acceleration of deuterium ions by the radiofrequency. The instantaneous neutron yield was limited by a strong influx of carbon impurity ions due to excessive, but local, wall heating.