The significance of various microorganisms as pathogens in septicemia has been changing over the years. This has been amply demonstrated by several authors especially in the interesting survey from Boston by Maxwell Finland covering a 30 years' period from 1935-1965. Dramatic changes were noted among the Gram-positives and this seemed to coincide with the introduction of antibiotics. However, since that time a continuous variation of microflora isolated from patients with bacteremia has been reported, which indicates the significance of a through analysis of isolated pathogens presently and in the future. The difference in etiology in various age groups has been strengthened and microorganisms, which were seldom found ten years ago, are now of major clinical importance in septicemia. There is no simple explanation to this phenomenon but rather reflects a summary of influences including changes of the host-microorganism interaction induced by different treatment procedures, survival of patients with diseases with a negative effect on the infectious response, new and better microbiological diagnostic procedures, etc. The possibility that the increased frequency of e.g. group B streptococcal infections reflects a change in 'Status epidemicus' must also be born in mind. The clinical significance of changes in the etiology of septicemia is obvious since the microorganisms have different virulence factors, influencing in various ways upon the host. These microorganisms may also have a different or changing antibiotic sensitivity pattern. It should be added, however, that the changes in antibiotic sensitivity pattern is a multifacetted problem involving factors like types of patients in a hospital, treatment regime, antibiotic use.