We can no longer accept classification of streptococci solely on the basis of hemolytic reactions or Lancefield agglutinations. While the "viridans" streptococci cannot be serologically differentiated, physiological separation of the species offers a satisfactory method of classifying human isolates. We review the microbiology of Lancefield group F and related streptococci (S. milleri, S. anginosus), emphasizing microbial ecology and current taxonomic considerations. A series of 28 patients infected with these organisms is presented. There was a striking male predominance in the series (6:1) as well as an obvious association with underlying diseases and/or antecedent trauma. The most remarkable feature of these pathogens is their predilection for abscess formation, confirming their overdue recognition as the most common "viridans" streptococcus recovered from abscesses within internal organs. We observed purulent disease of the nervous and skeletal systems, oral cavity, lung and pleural space, abdomen and subcutaneous tissues. Microbial synergy (i.e. polymicrobic infection) was not a requisite for abscess formation. Four of the five deaths in this series occurred in patients 60 years of age of older. Some degree of variability in antimicrobial susceptibility was noted, so speciation alone may not always provide sufficient information on which to base a therapeutic decision.