For detection of Streptococcus agalactia, C. equi, C. renale and C. diphtheriae (int ermedius) antibodies an in vitro technique (antihaemolysin potentiation test) was used. Ten isolated strains of Streptococcus agalactia and N.C.T.C. from the other species were studied. The prepared supernatant fluid from the different strains of the previous species and determination of their minimum potentiating dose (MPD) were done using t minimum haemolytic dose (MHD) of staphylococcal B. lysin. The possible neutrialization of the supernatant fluids by the different prepared ant Isera as well as comerica) antitoxin of c.diphtheriae and antistreptolysin were also studied.