IN these laboratories we have found tomato and potato suitable subjects for the study of the effect of streptomycin on host - pathogen relationships and have observed an interesting effect of this antibacterial substance on the course of development of infections due to Phytophthora infestans. Bonde1 has reported a similar effect from the absorption of terramycin hydrochloride through the cut stems of Katahdin potato plants, but failed to obtain an effect with streptomycin even at concentrations of 100 µgm.ml.