129 sera with known antibody titres against E. coli 026 and E. coli 055 strains were tested with the Abbot second generation anti-HTLV III recombinant screening assay. No difference in the O.D. values was found between sera with high, normal and low anti-E. coli titres. In addition, no false-positive reactions were observed with the anti-HIV negative sera containing E. coli antibodies in high titres in a Western blot assay in which recombinant env antigen was applied. These results suggest that E. coli assays in which E. coli-produced recombinant antigens are used.