This monograph deals with the sources of error commonly encountered in biologic assays of estrogens and androgens. There is presented in great detail the differences in results obtained with various technics of assay and the factors responsible for these differences. The author has demonstrated the common fallacies in comparing various sex hormones among themselves either in crude extracts or in pure forms on a biologic basis. Biologic assays may vary to a marked degree and may be quite unreliable unless numerous factors influencing the results are rigidly controlled. The clinical interpretation of results on urinary assays performed hurriedly and with insufficient equipment are, therefore, open to grave doubt. This monograph is an excellent contribution to the problems of the bio-assay of sex hormones and should be read by all those whose interests lie in this direction.