Small eyes (Sey) is a semidominant, homozygous lethal mutation in the mouse (Roberts, 1967). It is allelic with SeyH, a radiation-induced homozygous prenatal lethal which has been mapped on chromosome 2. The effect of the Sey mutation is apparently limited to the growth and differentiation of the presumptive lens and nasal placodes. Homozygous Sey/Sey embryos can be distinguished as early as 10.5 days post coitum (p.c.); the optic vesicles grow out, but the ectoderm does not give rise to a lens and nasal pits never form. Immunohistochemical studies show that the distribution of the extracellular matrix glycoprotein laminin is not significantly different in the cephalic region of Sey/Sey versus Sey/+ or +/+ embryos. Sey/Sey embryos develop to term but without eyes or nose, and die soon after birth. Further analysis of Sey/Sey embryos may throw light on the mechanisms underlying morphogenesis of craniofacial structures in mammals.