A complex interaction of host and microbial factors is unquestionably related to the pathogenesis of gram-negative rod bacteremia in neutropenic, immunocompromised patients. In this paper we summarize evidence that colonization of the gastrointestinal tract often precedes systemic invasion by klebsiellae and Pseudomonas aeruginosa, but that the factors directly responsible for the weakening of barriers to colonization remain poorly understood. Additionally, bacteremic isolates of Escherichia coli appear to segregate into commonly occurring groups by O and K antigens. A broadened investigation of E. coli surface (fimbrial) antigens indicates several common hemagglutination patterns of bloodstream isolates with various mammalian erythrocytes, but these patterns may also be strongly associated with commonly encountered O and K types. This epidemiologic and microbiologic information may be useful both in clinical management and in following measures to prevent infection in high risk immunocompromised patients.