Quiescent NIH 3T3 (left-hand panels) and gpl45trkB-expressing Z52-17 cells (right-hand panels) were incubated in the presence of 100 μM BrdUrd plus 5 μg/ml insulin and the following additions: 20% calf serum (A, B), no additions(C, D), 50 ng/ml NGF(E, F), a1:10 dilution of supernatants derived from COS cells expressing BDNF (G, H), or a 1:10 dilution of supernatants derived from COS cells expressing NT-3 (I, J). Cells were fixed after 24 hr and analyzed for immunofluorescence using a mouse anti-BrdUrd monoclonal antibody and donkey anti-mouse IgG conjugated with Texas red. Final magnification 290 x.