Previously described methods of phenotyping red cells sensitised with IgG using the indirect antiglobulin test required the dissociation of the coating protein. Based on an entirely different principle, Fab fragments of anti-human IgG (Fab anti-IgG) were used to block the antiglobulin binding sites on cell-bound IgG molecules, removing the necessity to dissociate them from the red cell. Fab anti-IgG was found to be effective in blocking interfering IgG on in vivo and in vitro IgG-sensitised red cells, permitting successful red cell phenotyping. Strongly IgG-sensitised samples which could not be fully neutralised by chloroquine diphosphate (CDP) or blocked with Fab anti-IgG alone could usually be phenotyped using a combination of both these methods. This new procedure may be of use in immunohaematology laboratories.