Blood of pigs and cows from Langford/Bristol (U.K.) and Hannover (F.R.G.) used in membrane feeding of tsetse flies was analysed for 21 trace elements; an influence of the location of origin was not detectable. Between the two host species a significant difference of the element content was established for bromine, copper, rubidium and zinc; if this observation is responsible for the different nutritive value of pig's and cow's blood it cannot be identified from the present experiment.