Injections of the B-subunit of cholera toxin (CTB) were made into the sciatic nerve of the rat. Following a survival of 2-3 days, the fluorescent antibody technique was used to show that CTB can be utilized as a highly sensitive immunocytochemically detectable transganglionic tracer for primary afferent fibers in the spinal cord. CTB-labeled fibers as well as fibers containing calcitonin gene-related peptide- (CGRP-) or substance P-like immunoreactivity were visualized simultaneously by using different fluorochromes. However, no double labeled fibers were found.