Islets of Langerhans cultured 7 days in vitro no longer contained any capillary endothelial cells, but their endocrine cells remained ultrastructurally normal up to 14 days. Vascular endothelial cells were also lost from cultured thyroid lobes, but more slowly. Thyroid endothelium was readily identified after 7 days of culture, although many cells appeared to be degenerating, and a few degenerating endothelial cells were still present after 14 days in culture. Erythrocytes and leukocytes in the lumina of thyroid vessels were observed to degenerate at about the same rate as the endothelial cells, while those in islet capillary lumina were largely washed out during isolation of the islets. Thyroid lymphatic endothelium and the numerous adipose cells present in this tissue also degenerated during the culture period. Follicle epithelial cells remained viable throughout the culture period, but the number of colloid droplets and endocytic vesicles they contained was markedly, decreased. Thyroid fibroblasts remained viable and appeared to enlarge and accumulate dense granules during culture. These cells were a prominent feature of thyroid lobes after 14 days of culture. Parathyroid tissue associated with the thyroid lobes showed viable endocrine cells but a loss of vascular endothelium after 14 days in culture. The loss of blood leukocytes and vascular endothelial cells in probably the major factor in the altered behavior of thyroid and islet allografts after culture in vitro.