Cell-lethal developmental mutations, which are presumed to affect the viability of all cells in a mutant embryo, have been distinguished from other development lethals on the basis of the results of parabiosis and transplant experiments. Premature death (p), previously classified as a cell lethal, does not survive parabiosis. However, transplants involving mutant eye, flank epidermis and primordial limb tissue all survived on a normal recipient. The mutant, therefore, cannot be considered a true cell lethal, though it suffers from serious and widespread abnormalities that cannot be corrected by parabiosis. In addition, transplants of mutant branchial mound tissue did not develop into normal gills on a normal recipient. These transplants were the only ones involving mutant endoderm, and their failure supports our hypothesis that the mutation leads to a specific endoderm defect.