Changes in patterns of soluble proteins of bean leaves after infection with bean rust (Uromycesphaseoli) were studied by electrophoresis in polyacrylamide pore gradient gels (4%-25% acrylamide). Four newly formed acidic proteins not present in healthy leaves were found in extracts of inoculated leaves of the resistant, hypersensitively reacting variety (017). Their mol. wts were estimated as 17 000 (P1), 27 000 (P/), 33 500 (P3) and 34 000 (P4) (in the absence of SDS). The most prominent protein, P1, could be detected already 1 day and proteins Pz-P4 2-3 days after inoculation; all increased in amount up to 8 days after inoculation. All new proteins were restricted to the hypersensitively reacting tissue. Changes were also detected for soluble basic pr6teins. Five bands which were faintly visible in healthy leaves of the resistant variety increased dramatically in intensity between the 4th and 8th day after inoculation with the fungus. Four new proteins (identical or very similar in size to those found in the resistant bean variety) were induced in non-inoculated leaves of the susceptible variety Favorit by infiltration of a polyglucan which acts as an elicitor of phytoalexin synthesis (1). Inoculation with the fungus 2 h after infiltration with the elicitor resulted in a successful infection, whereas an infiltration 3 days before inoculation caused total protection, correlated with the presence of the four new proteins.