Human Lesch-Nyhan cells, which are coupling and have gap junctions, were fused with mouse cl-lD cells, which are noncoupling and lack gap junctions. The resulting hybrid cells were coupling and had gap junctions while they contained the nearly complete complement of parent chromosomes. As the hybrid cells lost human chromosomes, clones appeared among the segregants, which had reverted to the noncoupling and junction-deficient trait of the mouse parent cell. The human cell appears to contribute a genetic factor to the hybrids that corrects the junctional deficiency of the mouse cell.