Virulent TGEV replicates primarily in the intestine and induces diarrhea; attenuated TGEV replicates in the intestine and the upper respiratory tract but induces no diarrhea; and PRCV replicates in the upper and lower respiratory tract, but induces only a subclinical infection (Table II; Frederick et al., 1976; Pensaert and Cox, 1989; Saif and Wesley, 1992) .