A small hemodialysis culture unit was developed which can be attached to an arterial-venous shunt and worn by an animal for days. The unit consists of a blood channel separated by a membrane from a dialysate chamber in which microbial or mammalian cells can be cultured. Bacterial multiplication proceeded first exponentially at the maximal rate and then arithmetically at a lesser, dialysis-limited rate. In a survey of 16 pathogenic microorganisms and five types of mammalian cell, results indicated that most of the aerobes grew well, but none of the obligate anaerobes grew at all. The separation of the culture from cellular and macromolecular host defense mechanisms allowed the cultivation of parasitic cells on an animal that was not naturally susceptible.