A simple blood agar plate medium for demonstration of the streptococcal serum opacity reaction.

Blood agar plates for demonstration of the streptococcal serum opacity reaction were prepared by pouring into Petri discs four layers, one above the other, in the following sequence: 1. agar; 2. horse-sheep blood mixed with agar; 3. porcine serum in agarose; and 4. agarose. The strains to be tested were streaked with a loop on the plates and a control medium was prepared as described above, but without porcine serum. After incubation for 18 hours at 37 degrees C, the streptococci producing opacity factor showed greenish zones around the colonies or else no haemolysis at all on the plates containing porcine serum, in contrast to a clear betahaemolysis on the control plates. Opacity reaction negative steptococci showed the same clear beta-haemolysis on both plates. Concordant results were obtained by this method and a standard method for demonstration of opacity factor.