A hydrogen maser was installed at the Harvard College Observatory-Smithsonian 84-foot radio telescope in Harvard, Massachusetts, for very long baseline interferometry (VLBI) experiments in October, November, and December of 1969. The maser is a compact unit (42 by 22 by 22 inches) of relatively low power consumption, designed specifically for field use; it is readily transportable. A precision 5-MHz crystal oscillator that is phase locked to the maser provides all frequency and time references for the VLBI receivers, recorders, and clocks. The maser was tuned against a rubidium standard by a rapid flux-switching technique; maser frequency resettability was estimated to be approximately ±3×10−13 with an over-all accuracy of approximately ±1×10−12. Loran-C was continuously monitored during the VLBI experiments, and the results of the Loran-C maser comparisons are included.