Abstract <p>Plants are considered as an alternative expression system for the production of valuable recombinant proteins–biopharmaceuticals. The transient gene expression method allows for obtaining high levels of target recombinant proteins within 1–2 weeks after infection of plant cells. However, the expression of the target transgene and the final yield of the recombinant protein depend on many factors: the efficiency of the expression cassette containing the transgene, stability of the target recombinant protein in a plant cell, <i>Agrobacterium</i> strain used to deliver a transgene into a plant cell, the host plant species producing the recombinant protein and its physiological state, exposure to certain chemicals, and physical environmental conditions. The review summarizes and discusses achievements related to the issue of increasing the yield of the target recombinant proteins, along with examples of valuable recombinant proteins obtained through transient gene expression.</p>