WASHINGTON — The Obama administration said Monday that it would investigate prescription drug coverage and other benefits offered by health insurance companies to see if they discriminated against people with AIDS, mental illness, diabetes or other costly chronic conditions.

The administration said it had become aware of “discriminatory benefit designs” that discouraged people from enrolling because of age or medical condition.

In a letter to insurers, administration officials said that a health plan could be engaging in unlawful discrimination if its list of approved drugs excluded all medicines needed to treat a particular condition, or if it restricted access to such drugs by charging large co-payments or requiring prior authorization.

The Centers for Medicare and Medicaid Services said it would focus on companies in the federal insurance marketplace. For each health plan, it said, it will try to determine the “estimated out-of-pocket costs associated with standard treatment protocols for specific medical conditions using nationally recognized clinical guidelines.” The conditions, it said, are likely to include bipolar disorder, diabetes, H.I.V., rheumatoid arthritis and schizophrenia.