A light string system is provided having a plurality of mono-color or bi-color light-emitting diodes electrically connected thereto. Each light-emitting diode has a generally elongated, hollow envelope mounted thereover, and the envelope is substantially filled with light-conducting optical spheres or even fragments. The envelope may be additionally filled with light-conducting epoxy, light-conducting liquid or light-conducting gas and sealed for improved light transmission and dispersion characteristics. The light-emitting diodes may include an improved base with light-emitting diode leads disposed approximately perpendicular to the axis of the envelope for bulb stability and for enabling the bulb to stand upright on the branches. Both the envelope and the optical spheres include light-conducting glass or plastic material. The spheres may be either hollow or spherical and may be of a uniform or a mixed size. Additionally, the balls may be large for stacking in single file within the envelope or centrifuged to the sides of the envelope with the center devoid of spheres. The light-emitting diodes may be adapted for use as Christmas tree strings and various AC and DC control circuits are provided for driving the light-emitting diodes, for blinking effects, for alternating between two colors, for alternating between three colors, for improving the length of the string and uniformity of intensity of illumination and the like. In a preferred embodiment of the Christmas tree string of lights, a master trunk line and a plurality of limb lines are provided. The limb lines plug into or connect to predetermined locations along the master trunk line for ease of assembly, replacement and the like. This string uses a series--parallel configuration wherein the upper and lower portion of the master trunk line and the individual light-emitting diodes within each limb line are connected in series while the limb line sets are connected in parallel with each other for enabling more lights to be illuminated with less power, for increasing the uniformity of illumination along the length of the string, and for minimizing light outage due to bulb burnout.