Pneumocystis carinii is the causative agent of P. carinii pneumonia (PCP), an opportunistic infection associated with AIDS and other immunosuppressed conditions. Although polyamine metabolism of this fungus has been shown to be a chemotherapeutic target, this metabolism has not been thoroughly investigated. Reported here is the effect of one polyamine analogue, N,N'-bis{3-[(phenylmethyl)amino]propyl}-1,7-diaminoheptane (BBS), on P. carinii. BBS inhibits the growth of P. carinii in culture, but at concentrations higher than those required to inhibit the growth of other pathogens. However, BBS is at least as active in an animal model of PCP as in other models of diseases studied. BBS causes some reduction in P. carinii polyamine content and polyamine biosynthetic enzyme activities, but the effect is less than that observed with other pathogens and very much less than the effect of the polyamine biosynthesis inhibitor DL--difluoromethylornithine. BBS enters P. carinii cells via a polyamine transporter, unlike all other cells that have been studied. P. carinii cells do not remove the benzyl groups of BBS, as is reported for mammalian cells. The most likely mode of action is displacement of natural polyamines. Overall, the activity of BBS provides further evidence that polyamines and polyamine metabolism are rational targets for the development of drugs to treat PCP. Because the details of BBS-P. carinii interaction differ from those of other cells studied, polyamine analogues may provide a highly specific treatment for PCP.