Containing formaldehyde, ethanol, isopropanol, or other alcohols
Quantity limits: No more than 30 ml of free liquid solution in each inner packaging and no more than one liter in the entire outer package.*
Inner packagings must be placed in a heat-sealed plastic bag.
Heat-sealed plastic bags must be placed in another heat-sealed plastic bag with enough absorbent material to absorb all of the contents of the inner containers. That bag is then put into a strong outer packaging with sufficient cushioning material.
The outer package is suitable marked "Scientific research specimens, 49 CFR 173.4b applies."
For complete packaging requirements and options see 49 CFR 173.4b(b).
See the regulation: 49 CFR 175.10(a)(23)
Tip: Full-strength "Formalin" is typically 35%-40% formaldehyde by volume. So a solution that is less than 25% Formalin would be less than 10% formaldehyde and not restricted. Leakproof packaging with absorbent material is still strongly encouraged.
*Liquids, gels, and aerosols in carry-on baggage are further limited to 100-ml (3.4 oz) containers at the TSA security checkpoint.