Sentence:  In all the above procedures , when the intermediate sera were diluted to 1 : 10 or 1 : 100 with 0.85 % saline , the specific and nonspecific fluorescence were not appreciably reduced , whereas , a dilution of the intermediate sera to 1 : <WSD>500</WSD> or diluting the * * f to 1 : 5 greatly reduced specific fluorescence . 
Meaning of the word marked with <WSD> tags:  denoting a quantity consisting of 500 items or units