Both species of broomrape tested were devoid of L-threonine dehydratase and L-serine dehydratase activity. When the seeds of three species of dodder were examined, threonine dehydratase activity was present in two but absent in the third. The enzyme could also not be demonstrated in the seed of a green mistletoe. Absence of threonine/serine dehydratase may be a factor necessitating or associated with the parasitic existence of broomrape. The significance of the presence of threonine dehydratase in some seeds and absence in some others is briefly discussed.