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The absorption of brighteners by living cells and their transport to subsequent growth is described. Brighteners are highly fluorescent, ultraviolet-absorbing compounds which appear to be essentially nontoxic, stable biological markers. They have been effectively absorbed by growing cultures of bacteria, yeasts, actinomycetes, and higher fungi, with active growth centers evidencing the greatest flourescence.

A brightener and a nonfluorescent ultraviolet absorber were found to be associated with an increase in the per cent of spore germination and cultural variation in a streptomycete and a Penicillium. When the brightener was combined with ultraviolet light, more variants were obtained than with either ultraviolet or brightener alone. This effect occurred at concentrations of brightener substantially greater than those used for fluorescent labeling.