Sodium iodoacetate at I m M was the best abiotic glyceollin elicitor of several compounds infiltrated into primary leaves of cv. Harosoy soybeans. Heterologous Pseudomonas spp. (pathogenic to plants other than soybean) elicited a visible hypersensitive response (HR) and caused considerable accumulation of glyceollin and related isoflavonoid compounds. However, the quasi-pathogen P. aeruginosa and the saprophyte, P. fluorescens, elicited neither a visible HR nor glyceollin accumulation when inoculated into soybean leaves. Glyceollin accumulation and visible HR were blocked when leaves were infiltrated with blasticidin S (BcS) at the time of, or up to 9 h after, inoculation with incompatible Pseudomonas spp. BcS also blocked glyceollin when leaves were infiltrated with sodium iodoacetate. Rifampicin-resistant (rif) mutant strains of incompatible race 1 P. syringae pv. glycinsa and the heterologous pathogen P. syringae pv. pisi, multiplied in BcS-treated soybean leaves at rates similar to compatible P. syringae pv. glycinea races, consistent with the hypothesis that the HR/glyceollin mechanism normally accounts for restriction of their populations. This could not be concluded with certainty, however, since BcS also allowed multiplication of P. fluorescens rif in soybean leaves. In addition to blocking resistance in inoculated leaves, BcS caused an increased loss of water and nutrients from uninoculated leaf cells. To circumvent the problem of nonspecific effects due to BcS, elevated temperature was used to break resistance expression. A hypersensitive reaction and glyceollin accumulation occurred if plants inoculated with P. syringae pv. glycinea race 1 were maintained at 22 °C, but a fully compatible reaction with no visible HR or glyceollin resulted when plants were incubated at 31 °C. Hypersensitive reactions and phytoalexin production occurred at both temperatures in response to sodium iodoacetate, P. syringae pv. glycinea race 6 or P. syringae pv. pisi, therefore indicating that race 1 but not race 6 of P. syringae pv. pisi is a naturally occurring, temperature sensitive strain in cv. Harosoy leaves. Streptomycin did not inhibit HR and glyceollin accumulation in leaves infiltrated with sodium iodoacetate but blocked visible HR and glyceollin accumulation if infiltrated within 2 h after inoculation with incompatible Pseudomonas spp. The experiments indicated that incompatible living but not streptomycin-inhibited bacteria are initially recognized by soybean cells sometime before 2 h after inoculation and that the HR and glyceollin accumulation are associated with resistance expression.