Supravital staining of spermatozoa: relationship of eosin concentration to the percentage of cells staining live. 
Supravital staining of human spermatozoa is a useful technique to assess semen quality.  We compared 3 concentrations of eosin (1, 2.5 and 5 per cent) for their effectiveness to differentiate viable and non-viable spermatozoa.  The percentage of viable cells determined by each concentration was compared as well as the percentage of cells estimated to be active.  The results indicate that the percentage of spermatozoa determined to be viable with the supravital stains can be altered by changing the percentage of eosin in the stains.  Use of 1 per cent eosin gave values that were significantly higher than the percentage of cells determined to be viable with 5 per cent eosin and the percentage of cells estimated to be active.  Better quality slides were produced with 5 per cent eosin, which provided values that correlated favorably with motility estimations.