Stems of cut rose flowers (Rosa hybrids L., cvs. Sonia, Ilona, Polka, and Frisco) were held in a sodium hypochlorite solution and then placed in distilled water or in a buffer at pH 6.0. After 2 days, many bacteria were found in the basal end of the stems, even when the number of bacteria in the water was below the detection limit. The hydraulic conductance of 5-cm stem segments was reduced whenever the number of bacteria exceeded =106 cfu/g fresh weight. Adding HQC or a buffer at pH 3.0 limited the number of bacteria in stems. Hydraulic conductance of the stems held in these solutions for 2 days was as in stems of freshly harvested flowers. Thus, HQC and low pH prevent vascular blockage by reducing the number of bacteria in the stems. No evidence was found for the hypothesis that HQC and low pH inhibit a stem-induced vascular blockage.