We studied thirty-five process and laboratory workers occupationally exposed to Paecilomyces varioti protein. The protein, used as an animal feed, is prepared industrially by fermentation in pulp waste liquor. Skin tests and radioallergosorbent tests (RAST) to detect specific IgE antibodies showed positive results to Paecilomyces antigen among the workers and among the atopic control persons not occupationally exposed to Paecilomyces protein. Also IgG antibodies to Paecilomyces protein could be detected in workers and in controls in similar amounts in both groups. In no worker could clinical symptoms due to Paecilomyces protein allergy be found. We conclude that positive skin tests and RASTs indicate that immediate hypersensitivity to Paecilomyces antigens occurs. Clinical allergy probably has been prevented by the closed manufacturing process, which keeps occupational exposure at a low level.