Crossreactivity to Dactylis glomerata, Festuca rubra, Phleum pratense, Anthoxanthum odoratum, Secale cereale, Zea mays, and Phragmites communis of IgE antibodies against Lol p I or Lol p V was investigated by means of RAST-inhibition. Within a group of sera the degree of crossreactivity was demonstrated to be highly variable. Individual sera were not always equally crossreactive to all pollen species. A high degree of crossreactivity for Group I allergens did not necessarily implicate the same for Group V. Group I and Group V representatives were found to be present in all eight species. It was demonstrated that within this group of grass species significant quantitative and qualitative differences exist, with respect to Group I and Group V allergens. Species with a low phylogenetic affinity to Lolium perenne, like Zea mays and Phragmites communis showed a very low degree of reactivity, even when measured with the most crossreactive sera. A higher taxonomic relationship however, did not always implicate a closer antigenic resemblance. Antigenically both allergens from Zea mays are more similar to Lol p I and Lol p V, than the analogues in Secale cereale.