Introduction Quarterly summary reports are prepared by the Health and Safety Laboratory (HASL) with the objective of presenting a current picture of the Strontium Program. It is hoped that these reports will aid investigators in relating their own work to that of others. Thus we urge other investigators to send recent results of their work to the Health and Safety Laboratory for publication in succeeding summaries. No attempt is made to interpret the data in these reports. This report, which up-dates HASL~51, presents data routinely reported by the Analytical Division of the Health and Safety Laboratory and four contractor laboratories - Nuclear Science and Engineering Corporation, , Isotopes, Incorporated, Radiochemistry, Incorporated, and Tracerlab, Incorporated. In addition,this issue includes data submitted by the U. S. Public Health Service and the Physikalisches Institut der Bundesforschung- sanstalt fur Milchwirtschaft; a Food and Drug Administration summary of their own work also appears. Omission of one phase of the program in a given quarterly period indicates that insufficient information has accrued to justify its inclusion in a given issue. Please note that data presented in these summaries are subject to revision and that changes in format may occur because of the dynamic nature of the programe