There are more than 100 low-frequency antigens (LFAs) which have been given International Society of Blood Transfusion (ISBT) numbers as members of systems, collections or the 700 series. In addition, there are a number of well-known (to reference laboratories) unpublished LFAs. The presence of an LFA was suspected when 2 sera were found to react with a single example of K homozygous cells. Anti-K reacting with K homozygotes was eliminated on testing with other KK cells. Testing of the reactive cell with antibodies to known LFAs and the reactive sera with cells known to carry LFAs failed to identify the specificity. A study on the family of the cell donor showed inheritance of the antigen in two generations. Further testing, which included immunoblotting and RFLPs, was carried out in Australia, the UK, Canada and the USA. By March 1993 all published LFAs had been excluded, and an application was made to the ISBT to have the antigen, SARAH, assigned a 700 series number. In April the number 700.052 was provisionally designated for this new antigen.