Two natural human interferon alpha preparations, (nHuIFN-μ [Cantell]) and (Nhuifn-μ [ISI]), were used for the oral treatment of cattle experimentally infected with Theileria parva parva. In the first experiment, 8 Friesian bulls were inoculated with a 1 in 10 dilution of a sporozoite stabilate of T. p. parva (Marikebuni) stock. Four of the cattle were treated daily with 1 international unit/kg body weight (i.u./kg bwt) of nHuIFN-μ (Cantell) from day –2 to day 8 p.i. None of the 4 calves given IFN developed clinical theileriosis, but 3 of the 4 control calves died of theileriosis while the fourth had a mild infection. Three of 4 treated calves and the 1 surviving control calf developed a detectable antibody response to T. p. parva schizont antigen but, on challenged with a 10-fold higher dose of stabilate, the surviving control animal and only 1 of the 4 treated calves proved to be immune. In a second experiment, 4 groups of 4 calves were inoculated with the same stabilate dilution. Three treatment groups were given either 1 i.u. nHuIFN-μ (Cantell), 1 i.u. nHuIFN-μ (ISI), or 10 i.u. nHuIFN-μ (ISI)/kg bwt from day –2 to day 8 p.i. once daily and the fourth group were controls. Clinical theileriosis occurred in 2 controls, 2 calves given 10 i.u. nHuINF-μ (ISI), 1 calf given 1 i.u. nHuIFN-μ (ISI) and no calves given 1 i.u. nHuIFN-μ (Cantell)/kg bwt. Of these, 2, 1, 0 and 0 cattle died in the respective groups. All the surviving cattle proved to be immune on homologous challenge with 10-fold higher dose of stabilate except the 2 cattle which did not develop high antibody responses. A third experiment using an undiluted challenge of T. p. parva (Muguga) sporozoite stabilate (10α) on 8 steers. Four steers were treated with 1 i.u. nHuIFN-α (Cantell)/kg bwt and 4 were controls. All calves developed acute theileriosis and the experiment was terminated. Cells of the C2 lymphoblastoid cell line, infected with T. p. parva (Muguga) schizonts, were cultured in vitro with various concentrations (0-01-100 i.u./ml) of nHuIFN-a (Cantell). The IFN appeared to have no effect on host cell or parasite developmental variables when compared to untreated control cultures.