The aryl arsonic acids, 4-hydroxy phenylarsonic acid, 3-nitro 4-hydroxy phenylarsonic acid and their Na salts, when given at relatively non-toxic doses prior to infection, have repeatedly reduced or completely eliminated hemorrhage and mortality in test groups of chickens infected with the coccidium Eimeria tenella. A prepn. containing Na 4-hydroxy phenylarsonate, Na and ammonium phenolsulphonates, boric acid and lactose has shown high efficacy when the conc. of the arsenic compound in water was 0.0269%. In 3 expts. using 252 treated chicks and 252 controls, the mortality from cecal coccidiosis was 2 and 83, respectively. Similar results have been obtained with phenol-sulfonate-lactose-boric acid prepns. containing 3-nitro 4-hydroxy phenylarsonic acid where the conc. of the As compound in the water was much lower than was necessary when 4-hydroxy phenylarsonic acid was used. When the conc. of 3-nitro 4-hydroxy phenylarsonic acid in such prepns. was approx. 0.00256% in the water, marked growth stimulating properties were noted. In numerous expts., statistical analyses have shown highly significant dif- ferences between the mean gains of treated and control chicks.