The present invention relates to novel methods for detecting a member of a known binding pair in a sample, including a cell, where one member of the pair (termed the “receptor”) is expressed by a bacteriophage, which phage is then used to detect the presence of the other member of the pair (termed the “ligand” or “target”). Rather than detecting the binding of the phage using antibody-based technology, the present invention relates to detecting marker molecule associated with the phage. In one aspect, the invention relates to identifying an antigen-bearing moiety (e.g., a red blood cell antigen) of interest present on a cell, e.g., a red blood cell, using antibody-displaying bacteriophage, as well as detecting anti-red blood cell auto- or alloantibodies and/or complement in a sample, using antiglobulin reagent-displaying bacteriophage and detecting a marker molecule associated with the phage. In one aspect, the phenotype of the phage is not linked with the genotype of the phage.