Summary. Among women immunized to the Rh(D) antigen, the titre of anti‐D measured by the indirect anti‐globulin (TAG) technique falls, on average, to half of the original value in 5–7 years after a pregnancy, but there is a wide range of individual variation. This range is specified. The fall is greater when the initial titre was high. The average titre of saline anti‐D activity falls much more rapidly, although in a minority of individuals this class of antibody persists for many years. The saline anti‐D titre of some women may rise following pregnancy.