In order to visualize the microtubules present in a culture of 3T3 fibroblasts, the cells were immunofluorescently labeled with anti-tubulin mouse monoclonal primary antibodies followed by goat anti-mouse Fab fragments conjugated to Alexa Fluor 555. In addition, the cells were labeled for DNA in the nucleus with the red-absorbing dye TO-PRO-3. Images were recorded with a 60x oil immersion objective using a zoom factor of 3.0 and sequential scanning with the 543-nanometer line from a green helium-neon laser and the 633-nanometer line of a red helium-neon laser. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.