The features of B-cell tolerance induced in mice with the chemically reactive hapten 2,4,6-trinitrobenzenesulphonic acid were investigated after various antigenic challenges. A complete abolition of the IgG response was observed after challenge with trinitrophenol (TNP)-coupled haemocyanin (TNP-KLH) associated or not with Escherichia coli lipopolysaccharide. Carrier immunization with KLH (3 micrograms) followed by challenge with TNP-KLH led to a normal IgG anti-TNP response. However, tolerant mice given 3 micrograms TNP-KLH (instead of KLH) and a repeat injection of TNP-KLH exhibited a depressed IgG anti-TNP response. These results are discussed within the framework of tolerance induction in adult mature B cells.