Biological products are considered to be those that cannot be completely characterized by chemical and physical methods. The control of biological medicinal products made by recombinant DNA technology, like those produced by established techniques, is thought to require attention to "in-process' control. This offers an economical and efficient way of assessing the quality of the end product. Evaluation of the starting materials and the manufacturing process may therefore be as relevant to ensuring the safety, purity and potency of biologicals as tests carried out on the final product. Of course, identification and characterization of the product will be essential and should encompass several different physico-chemical properties of the molecule. As yet there is little experience of recombinant DNA technology on a manufacturing scale or of the control problems which may be encountered. Currently proposed requirements should therefore be regarded as flexible and subject to alteration as experience of the production and use of these substances increases.