We report on two patients with actinomycosis. The first case was a 45-year-old cachectic man with extensive bilateral pulmonary infiltrates. The lesions remained unclear for 18 months and only open lung biopsy with microscopic and cultural evaluation led to the diagnosis of actinomycosis. In the second case, classic cervico-facial actinomycosis in a 69-year-old farmer is described. Diagnosis was established on the basis of microscopic findings of "sulphur granules" eroding the mandible. Furthermore, the clinical presentation, with an indurated swelling of the jaw and intraoral fistula formation, was typical. The patient had a severe combined aortic valve lesion and died after ventricular fibrillation during hospitalization. Diagnosis could not be confirmed by culture, probably due to antibiotic prophylaxis against endocarditis during tooth extraction on the first day of admission. Diagnostic difficulties and microbiologic aspects are discussed, with special focus on the rare species of Actinomyces meyeri which was cultured from biopsy specimens from the lung of the first patient. So far this species has been described in only 13 patients.(ABSTRACT TRUNCATED AT 250 WORDS)