Review of data for a ten-year period on culture vs. guinea pig inoculation for the diagnosis of tuberculosis indicates that animal inoculation was far superior to culture. This was consistently true each year. During this time, in 53 (14.8%) of 358 specimens the disease would not have been diagnosed if only cultures had been used to isolate the organism. The 53 specimens represented 38 patients. This contrasts with five specimens that were positive by culture only. The data indicate that both culture and guinea pig inoculation should be used for detecting mycobacteria. Cultures are necessary for isolating the atypical mycobacteria frequently found in lesions clinically and histologically resembling those due to tubercle bacilli. Care should be taken to have the guinea pigs on a diet free from antibiotics, which might give the effect of prophylactic chemotherapy.