Rubella antibody titers were assayed from 191 blood samples taken during seven years (1973–1979) from a human female who had contracted German measles in 1965. The antibody dilutions, transformed to integers, fit a first‐order harmonic function with an assumed period of 365 days. The levels about which the titers oscillated decreased from 1973 to 1975 and remained stable thereafter. Amplitudes averaged 31% on either side of the level of oscillation. At certain times of the years 1977–1979 at the trough of the rhythm the subject would have been considered to be lacking in immunity against rubella despite history to the contrary. This example of a long‐term, circannual rhythm in an immunologic assay supports previous suggestions that immunity cycles over the seasons.