One class of potential acquired immunodeficiency syndrome therapeutics are derivatives of recombinant CD4 (rCD4). Therefore, the present investigations use in vivo techniques to measure the rate at which [3H]rCD4 is transported through the blood-brain barrier (BBB). In addition, the binding of labeled rCD4 to isolated human and bovine brain capillaries is measured. These studies show that [3H]CD4 is removed rapidly from the bloodstream with a half-time of 12.6 +/- 0.9 min. The volume of distribution (Vd) of the protein in brain increases with time and reaches a Vd that is 11.1 +/- 1.1-fold greater than the brain Vd of plasma marker, native rat serum albumin. In addition, [3H]rCD4 is extracted rapidly by the kidney and the ratio of rCD4 Vd to native rat serum albumin Vd in the rat kidney reaches 99 +/- 5 at 60 min after i.v. injection. rCD4 is shown to undergo transcytosis through the BBB using an internal carotid artery perfusion/capillary depletion method coupled with gel filtration fast protein liquid chromatography. In conclusion, these studies report the unexpected finding that rCD4 is transportable through the BBB. rCD4 is a cationic protein and the mechanism of rCD4 transport through the BBB may be analogous to the absorptive-mediated transcytosis of other polycationic proteins.