Host-parasite specificity between zoospores of race 0 or race 1 of Phytophthora infestans and tuber tissues of Rishiri (R1) or Irish Cobbler (r) potatoes was examined in vitro. Proteinaceous substances in the 20, 000×g supernatant from potato tissue homogenate bound to substances in the 10, 000×g precipitate from homogenates of both race 0 and rece 1 zoospores. The binding reaction was less with Irish Cobbler than with Rishiri. The substances in host tissue which bound to substances from zoospores were found primarily in the 100, 000×g precipitate of the 20, 000×g supernatant. The binding reactions were inhibited by substances in the germination fluid or in the supernatant of homogenates of compatible zoospores. The significance of these in vitro interactions is discussed in relation to host-parasite specificity.