Phytohemagglutinin lectin (PHA) derived from red kidney bean (Phaseolus vulgaris) causes bacterial and protozoal colonization of the rat small intestine. To provide additional insights into this phenomenon we have studied the time course and population dynamics of microbial colonization of the major aerobe--facultative anaerobe groups which characterize this microflora. Compared with controls, PHA caused proliferation of a consistent adherent microbial flora in the jejunum (P less than 0.01). The predominant bacteria identified were Escherichia coli. a Streptococcal sp., and Lactobacillus. Escherichia coli isolates expressed no predominant serotype or fimbriae; none elaborated heat-labile or heat-stable toxin. Both E. coli and Streptococcal sp. populations increased within 24 h of PHA feeding and were sustained during further exposure to PHA (P less than 0.05). On reversion to a control diet, coliform counts fell progressively within 24-48 h and continued to decline, whereas gram-positive rod and coccus flora became the more prominent colonizers through days 1 to 4 of the reversion.