Cultures of an isolate of Penicillium verrucosum var. cyclopium, obtained from stored maize in an area of Balkan (endemic) nephropathy—Vratza, Bulgaria—has consistently induced renal tubular lesions when force-fed to rats for 20 days. The lesions, confined to the lower reaches of the proximal convoluted tubules (pars recta and junctional zone), closely resemble the tubular changes in patients with Balkan nephropathy. Preliminary evidence suggests that this nephrotoxin-producing strain of P. verrucosum var. cyclopium may be implicated in the ætiology of Balkan nephropathy.