The detergents 1-0-n-octyl-beta-D-glucopyranoside (OBG) and sodium n-dodecyl sulphate (SDS) have been used to extract blood group substances from human erythrocyte membranes for detection by enzyme-linked immunosorbent assay (ELISA). The effect of detergent concentration on the extraction process and detection by ELISA have been investigated. Detergent extraction increased the ELISA response relative to response from membrane suspensions approximately 1000-fold. Optimum responses occurred using detergent concentrations near the critical micelle concentration (cmc) for OBG and below the cmc for SDS. High detergent concentrations interfered with the ELISA but this effect was reduced by dilution of the extracts before adsorption of antigen on the microtitre wells. The interference effects of detergent on ELISA were also investigated using ovarian cyst glycoproteins as antigen. It was found that detergents inhibit the assay at the initial stage by competing with antigens for adsorption sites on the microtitre well surface and that subsequent detergent can displace pre-bound antigen. The results are discussed in terms of detergent binding to proteins (and glycoproteins) in relation to free (unbound) detergent concentration.