Homozygous transgenic mice are viable, fertile, normal in size, and do not display any gross physical or behavioral abnormalities. These mice express GFP in all tissues examined. Expression levels vary between certain hematapoietic cell types. GFP expression is uniform within a cell type lineage and remains constant throughout development. T cells have a 2-fold higher GFP expression than CD19+B220+ B cells or peripheral blood cells. Leukocytes and red blood cells from homozygous transgenic mice fluoresce at approximately twice the level of cells from hemizygous mice.