Pollens of related species usually share common antigens and are cross-reacting but this is not always the case. The cross-allergenicity of two species of Urticaceae pollens (Parietaria and Urtica, nettle) was sought by means of RAST-inhibition and isoelectric focusing. A preliminary experiment in forty-two patients investigated by means of skin tests and RAST showed that most patients were either allergic to Parietaria or nettle and that both reactivities were uncommonly associated. The results of RAST-inhibition confirmed the total absence of cross-reactivity between these two related species. IEF patterns indicated that most proteins are in the acidic range of pH and did not reveal large discrepancies. Parietaria and Urtica pollens, although belonging to the same family and having close microscopic patterns, have very different allergenic activities. This study demonstrates that cross-allergenicity cannot be predicted.