Evaluation of glial cell migration following transplantation can be difficult as the force of the injection itself may cause the cells to become immediately dispersed. In this study we evaluated the extent of spread of cells after injection of 1 microliter of a dissociated cell suspension (50,000 cells/microliter) into the dorsal columns of the thoracolumbar spinal cord in the neonatal myelin-deficient (md) rat. Spinal cords were examined at 0, 4, and 24 h after injection to determine the dispersion of cells away from the initial site of deposition. Examination of skip-serial sections collected at 50-microns intervals rostral and caudal to the site of transplantation showed that the injection could result in a spread of transplanted cells up to 1,600 microns. Migration should therefore be defined as the detection of cells beyond the rostral-caudal boundaries defined by the injection deposition. Cell dispersion should be taken into account when evaluating the results of migration in previous and future experiments concerning glial cell transplantation.