The green fluorescent protein (GFP) is responsible for the green bioluminescence of the jellyfish Aequorea victoria. Many classes of GFP mutants exist that display modified fluorescence spectra and an increased extinction coefficient. We produced transgenic mouse lines with an 'enhanced' GFP (EGFP) cDNA under the control of a chicken beta-actin promoter and cytomegalovirus enhancer. All of the tissues from these transgenic lines, with the exception of erythrocytes and hair, were green under excitation light. The fluorescent nature of the cells from these transgenic mouse lines would facilitate their use in many kinds of cell transplantation experiments.