Testimonial: Bioactivity was tested by culturing mouse ES cells either on feeders in complete medium or without feeders in a conditioned medium. In both conditions ISOkine™ LIF was added using concentrations down to 0.1 ng/ml. All concentration conditions resulted in nice looking colonies of pluripotent mouse ES cells. When the ES cells were deprived of LIF they spontaneously differentiated and were able to form embryoid bodies in hanging drops. Dr. Guðrún Valdimarsdóttir, Dept. of Biochemistry and Molecular Biology, Faculty of Medicine, University of Iceland.