A biochemical study of fourteen cases of multiple myeloma with Bence-Jones proteins present in the urine, is made. Bence-Jones proteins have several properties in common which distinguish them from the other normal or abnormal serum proteins. Thanks to their low molecular weight, they pass through the kidney and can be easily characterised in and extracted from the urine. Their electrophoretic mobilities are situated between those of the β and γ2 globulins in the serum. Complete thermosolubility at 100° C is not an absolute criterium for their identification. The presence of non-soluble fractions at boiling point does not necessarily imply the presence of other proteins. The amino acid content of a given Bence-Jones protein as determined by Moore and Stein’s method does not seem to differ appreciably according to the process used to extract it from the urine. The comparison of our results concerning the amino acid composition of electrophoretically differing Bence-Jones proteins shows an appreciable difference. The electrophoretic mobility of the β globulins in the Bence-Jones proteins is ascribed to a larger proportion of aspartic acid. The important quantity of threonine and serine is confirmed and the absence of hexoses or hexosamines is determined in all our samples. Methionine appears to be absent in the γ2 globulins of Bence-Jones. A completely thermosoluble Bence-Jones protein, proved to remain homogeneous by ultracentrifugation and electrophoresis, and to keep its characteristics in four different samples collected from the same patient over a period of seven months. Its amino acid content also remained the same. This observation ought however to be repeated in several similar cases in order to know whether every patient produces a Bence-Jones protein with its own characteristics in composition and properties and whether the latter are always kept uniformely constant throughout the course of the disease.