Culture media intended for in vitro diagnostic use—such as media used to identify pathogens in clinical specimens—may be regulated by FDA as Class I or Class II medical devices under 21 CFR Part 866. General-purpose research or industrial fermentation media not labeled for diagnostic use typically fall outside device regulation. Importers should confirm the product's intended use and FDA registration status before entry.

Heading 3821 covers prepared, ready-to-use or ready-to-reconstitute formulations designed specifically for microorganism cultivation. Pure agar as a gelling agent classifies in the chapter on seaweed extracts, and individual amino acids or vitamins classify in their respective chemical headings. Once those ingredients are combined into a formulated medium for microbial growth, the mixture moves to 3821.