Ron, Your problem sounds like one we had with our LSR whereby the laser1-excited PI of one cell would leak into the APC detector at the red laser time point of the previous cell. This random occurrence was cell rate dependent; the faster the cell rate, the greater the chance of a coincidence and the higher the percentage of spurious APC "positives". The (completely effective) fix was a modified APC channel optical filter holder, supplied by BD, which had a slightly lower pinhole and better blocked any laser1 spillover (from *any* cell) just by virtue of the geometry. A quick workaround may be to tape over the top half of your APC pinhole. Frank Battye WEHI Cytometry
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