The clinically known sensitive period of rubella cataract was studied in vitro by infecting 79 human eye rudiments from embryos aged 4-10 wk with rubella virus. The course of the infection was followed by histological and indirect immunofluorescence methods. Of the rudiments, 12 pairs were in the lens placode or open-lens-vesicle stage, 40 already had closed lens vesicles and in another 27 closed-stage pairs an incision was made in the lens capsule before infection to allow the virus to enter the lens. Uninfected controls differentiated well in vitro for 4-6 wk. The eye rudiments infected in the open-lens-vesicle stage showed lens fiber destruction and viral antigens within the lens. No damage or viral antigens were detected in rudiments infected in the closed stage unless the lens capsule was incisedmwhen this was done, however, fiber damage ensued and viral antigens appeared. The lens capsule was concluded to form a protective barrier around the sensirive fibers at the time of closure of the lens vesicle, confirming the earlier hypothesis and clinical findings.