Motorneurons and macrophages have been isolated and identified in primary cultures from adult frog (Rana pipiens) spinal cord. Time-lapse video microscopy revealed that during the first two weeks migrating macrophages contact the growth cones of motorneurons. As they continue to migrate, the motorneuron processes elongate in close association with the moving macrophages. Elongating motorneuron processes are thereby brought into contact with other motorneurons and networks are formed. At later stages, the macrophages die but the motorneurons and the networks survive for at least another two weeks. These experiments show that macrophages can promote a directed elongation of motorneuron processes and suggest that they play a similar role during regeneration in vivo.