To determine the effect of agar upon the germicidal potency of quaternary ammonium salts, 0.2 per cent agar was included in the germicide‐bacteria mixtures during the test period. Standard germicide tests without agar were included as controls. When the four quaternary ammonium salts were tested in the presence of 0.2 per cent agar, germicidal dilutions were much lower than germicidal dilutions in the absence of agar. The neutralization effect of agar probably accounts for this discrepancy. Standard tests for antiseptic potency call for evaluation of creams, ointments, powders, etc., by agar cup plate methods. It is evident that such procedures are inappropriate for preparations which contain quaternary ammonium germicides.