The widespread application of DNA techniques in medicine and biology has allowed the typing of human leukocyte antigens (HLA) at the molecular level. Comparative studies between serological and molecular biology methods have shown the existence of null alleles, which code HLA antigens with low or no cell surface expression. Null alleles are not detectable by standard serological typing methods and may be overlooked /or incorrectly assigned by available DNA methods. Although null alleles are infrequent in human populations, they should not be ignored. Errors in typing of null alleles may cause complications in the evaluation of HLA matching of donor / recipient pairs that were originally considered HLA-compatible. The detection of null alleles and their frequency in various populations requires typing of a large number of individuals by both serological andDNA-basedmethods. Knowledge of the haplotype(s) associated with null alleles may be helpful for their identification. For this purpose, it is necessary to perform family studies.

        Key words: histocompatibility antigens, null alleles, polymorphism, transplantation.