The new pharmaceutical and diagnostic compositions comprise antibody-aggregates and hydrophobic drug or marker molecules. The drug or marker molecules are solubilized inside micelle-like antibody-aggregates. In a process for preparing such types of compositions, hydrophobic residues are attached to the antibodies in the presence of a surface active agent. After removal of this agent the hydrophobic drug or marker molecules are solubilized inside the resulting antibody aggregates. A method for targeting molecules towards specific cells and sites within a living body is also disclosed.