Efficient detection of soft agar grown colonies using a tetrazolium salt

The use of 2-(p-iodophenyl)-3-(p-nitrophenul)-5-phenyl tetrazolium chloride at concentrations of from 0.25-1.0 mg/ml, when added to plates containing soft agar grown colonies, colors the cell clusters a deep brick red against a colorless background. This greatly facilitates the quantitation of such colonies since clusters of cells containing as few as 16-32 cells are visible macroscopically. Incubation for 20 h is optimal although coloration takes place as early as 6-8 h.