We describe the development and persistence of severe immunodeficiency in a previously healthy young woman shortly after she was deliberately injected with blood that was drawn from a patient with AIDS. The heterogenous populations of human immunodeficiency virus (HIV) in donor and recipient were as closely related as those reported in previous cases of proven transmission. The relatively large proportion of non-syncytium-inducing (NSI) clones in the recipient suggests a selective but not complete suppression of syncytium-inducing (SI) viruses. The continuous presence of SI viruses might explain the severe immunosuppression that persisted once the recipient seroconverted. A codon 215 mutation (indicative of zidovudine resistance) was present in SI and NSI clones of the donor and in NSI clones of the recipient. The relative increase in codon 215 resistance mutation in the absence of zidovudine therapy was secondary to the increase in NSI clones. Findings in this case suggest that qualities of an inoculum and/or the route of transmission are important determinants in the subsequent clinical course of HIV disease.