Abstract Aphids ( Myzus persicae Sulz.) often transmitted potato virus Y (PVY) after probing through membranes into sap extracts made in solutions containing ethylenediametetraacetate (EDTA) and sodium diethyldithiocarbamate (Na-DIECA) but rarely did so when these chelating agents were omitted. Aphids did not transmit from purified virus preparations alone but did when they were first mixed with the supernatant obtained by ultracentrifuging a fresh sap extract of PVY-infected leaves; the supernatants alone were practically noninfective and aphids never transmitted virus from them. Aphids given separate access to supernatant and purified virus transmitted when they probed supernatant before virus but not when they probed virus first. Supernatants from extracts of PVY-infected leaves also helped aphids acquire henbane mosaic, tobacco severe etch and potato aucuba mosaic viruses. The results suggest that sap of plants infected with PVY contains some component, other than the virus particle, that is needed for virus acquisition by aphids probing through membranes.