RANDOM association of VL and VH repertoires contributes considerably to antibody diversity¹. The diversity and the affinity are then increased by hypermutation in B cells located in germinal centres². Except in the case of 'heavy chain' disease³, naturally occurring heavy-chain antibodies have not been described, although antigen binding has been demonstrated for separated heavy chains⁴ or cloned VH domains⁵. Here we investigate the presence of considerable amounts of IgG-like material of M_r 100K in the serum of the camel (Camelus dromedarius)⁶. These molecules are composed of heavy-chain dimers and are devoid of light chains, but nevertheless have an extensive antigen-binding repertoire, a finding that calls into question the role of light chains in the camel. Camel heavy-chain IgGs lack CH1, which in one IgG class might be structurally replaced by an extended hinge. Heavy-chain IgGs are a feature of all camelids. These findings open new perspectives in the engineering of antibodies.