A new bioassay for A.C.T.H. has been devised utilising non-proliferative organ culture of segments of guinea pig adrenal. The cultured adrenal is exposed to standard or unknown A.C.T.H. and sections stained for the presence of reducing groups. The intensity of staining in the zona reticularis is measured by scanning and integrating microdensitometry and is inversely proportional to the logarithm of the concentration of A.C.T.H. in the culture medium over the range 0.0027–27 pg/ml.
Preliminary results using the assay, which greatly exceeds others in sensitivity, on human plasma are similar to those obtained with standard techniques.
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