Porcine interferon (POIFN)-alpha prepared in primed peripheral blood leukocyte cultures induced with Newcastle disease virus and POIFN-beta from PK-15 cell cultures induced with polyinosinic:polycytidylic acid were partially purified by precipitation with potassium thiocyanate and anion exchange chromatography. Mean purification factors in terms of units of POIFN per mg of protein, of 37 and 12 were obtained for POIFN-alpha and POIFN-beta respectively. In yield reduction assays in swine testis and pig kidney cell cultures, POIFN-alpha and POIFN-beta had greater antiviral activity against vesicular stomatitis virus than against transmissible gastroenteritis virus (TGEV). The antiviral effects were greater at higher concentrations of interferon (IFN), and when the IFN treatments were continued postinfection. Porcine interferon-beta showed greater antiviral activity against TGEV than POIFN-alpha, but this may have been partly due to cytotoxicity. There were no major differences in the antiviral activities of crude and partially purified IFN preparations. Both types of IFN showed antiviral activity against TGEV in yield reduction assays in porcine intestinal explant and intestinal epithelial cell cultures. Crude POIFN-beta was found to be rapidly cytotoxic, especially in porcine cells, and some fractions of partially purified POIFN-beta were also cytotoxic. The cytotoxicity of POIFN-beta was partially neutralized by antibodies against human IFN-beta, but human IFN-beta was not cytotoxic for porcine or bovine cells.