This report describes a feasibility study concerning the use of a visible diode 
laser for two important fluorescence applications in a flow cytometer. With a 3 
mW 635 nm diode laser, we performed immunofluorescence measurements using the 
fluorophore allophycocyanin (APC). We have measured CD8 positive lymphocytes 
with a two-step labeling procedure and the resulting histograms showed good 
separation between the negative cells and the dim and the bright fluorescent 
subpopulations. As a second fluorescence application, we chose DNA analysis with 
the recently developed DNA/RNA stains TOTO-3 and TO-PRO-3. In our setup TO-PRO-3 
yielded the best results with a CV of 3.4%. Our results indicate that a few 
milliwatts of 635 nm light from a visible diode laser is sufficient to do single 
color immunofluorescence measurements with allophycocyanin and DNA analysis with 
TO-PRO-3. The major advantages of using a diode laser in a flow cytometer are 
the small size, the low price, the high efficiency, and the long lifetime.