In three cases, two of which regarded exudative pericardium inflammation and one of lung abscess, biopsy revealed negative bacteriological results. Blood cultures resulted in culture of anaerobic Gram-negative bacteria from Propionibacterium genus. Precise chemo- geno-taxonomic studies were performed which confirmed generic identification of isolated strains. These were following: in cell wall--L-DAP, alanine, glycine, galactose, glucose, mannose; lack if mycolic acids; %GC 58-59. Basing on additional bacteriological studies it was established that two strains belong to the genus Propionibacterium acnes (type I), while third strain appeared to be Propionibacterium propionicum (before Arachnia propionica). These results confirm growing participation of relative pathogenic bacteria in opportunistic infections. It suggests necessity of blood cultures in cases subjected to prolonged therapy, especially when immunosuppression is involved.