The involution of the bursa of Fabricius in White Leghorn chickens and the transport of colloidal carbon in the involuting bursa were studied on light microscopy. In chickens, from newly hatched to 23.5 weeks of age, the bursa was weighed, its histology was described, and the mitoses in the cortical and medullary compartments of the lymphoid follicles were counted. Decrease in bursal weight and in the number of mitoses were observed between 10 and 16 weeks of age. During week 17.5, the follicle-associated epithelium began to lose its endocytic capability, and mucin droplets appeared in the follicular medulla initiating the large mucoid cysts that were seen in the later phases of involution. The involutionary process was almost completed by week 23.5, when the bursa appeared as a fibrotic residue without intact lymphoepithelial structures. The particulate tracer used, colloidal carbon, was not observed to be transported to other organs from the bursa. The present study gives further support to the idea that after completing its central immune function in 6 weeks [21] the bursa, as estimated histologically, still possesses capacity to function like a peripheral lymphoid organ until 16 weeks of age.