T HE Treponema pallidum immobilization (TPI) test described by Nelson (1,2) has been modified by many of the laboratories which perform this test (3). Controls for the originally described TPI test included a "quantitatively titered syphilitic serum." At a meeting called by the Division of Venereal Disease, Public Health Service, in Washington, D.C., February 1, 1952, to discuss the TPI test, several participants expressed the need for a common, reactive serum, available to all laboratories, by which sensitivity levels of the TPI test could be measured from day to day within a laboratory and also between laboratories. Later that year such a control serum was distributed by the Public Health Service's Venereal Disease Experimental Laboratory to the Venereal Disease Research Laboratory and to other laboratories. This serum was frozen and sent by airmail in packages with solid CO2. Experiments conducted in 1952 (4) indicated that whole serum, to be used as quantitative reactive controls for the TPI test, could be distributed to distant laboratories in the liquid state without added preservatives or refrigeration. In March 1953 freeze-dried ampoules of the first international TPI control serum were distributed to 25 laboratories by the World Health Organization (5), and in February 1954 ampoules of the second international control serum were sent to 35 laboratories. Results of testing with these serums are reported and analyzed in a 1956 WHO report (6) in which the following statement is made regarding the use of a quantitative serum in the TPI test: " In accordance with experiences from studies on reagin tests, the use of a reference (or standard) serum is recommended. In this way the effect of the variations of the titer levels between the laboratories as well as the temporal time variations within laboratories may be reduced." At the Venereal Disease Research Laboratory a reactive serum has been used as a quantitative control for the TPI test since July 1952. Since then the serum has been available to all TPI testing laboratories in the United States, under the following conditions: * A single serunm is to be common to all TPI testing laboratories, and this serum is to be replaced later by another serum that will be sent to all participating laboratories at the same time. The supply of serum on hand will suffice for at least 2 years. * All participating laboratories, including the Venereal Disease Research Laboratory, will keep a record of quantitative results obtained with this control serum each time it is used. A report containin-g these findings will be forwarded to the Venereal Disease Research Laboratory twice each year, in July and in January, where it will be duplicated and forwarded to all other participating laboratories without analysis or comment. The Venereal Disease Research Laboratory mailed the first shipments of serum, in quantities sufficient for approximately 6 months, to eight laboratories on January 20, 1958. The distribution of the serum is a continuing function of the Venereal Disease Research Laboratory, but only the results of testing for the 21/2-year period January 1958-June 1960 are included in this report, since a single lot of Afr. Harris and Dr. Brown are with the Communicable Disease Center, Public Health Service,, Atlanta, Ga. Mr. Harris is director, Venereal Disease Research Laboratory, Venereal Disease Branch, and Dr. Brown is chief of the branch.