Inclusion Criteria:

• Male and female subjects who are 18 years of age or older.
• Subjects in Treated Group:
• Subjects must have received linezolid 600 mg BID for two months or greater and be currently on drug (or have received linezolid within 7 days of baseline evaluation).
• Subjects who have current signs or symptoms compatible with linezolid toxicity (i.e. optic or peripheral neuropathy) may be enrolled in the study if they are on linezolid at time of baseline evaluation (or have received linezolid within 7 days of baseline evaluation).
• Linezolid may be discontinued at any time at the primary physician's discretion and remain on the study.
• Women of childbearing potential must use adequate contraception
• Subjects in Control Group:
• Subjects will have a diagnosis similar to patients in the treated group and similar important co-morbidities and epidemiologic factors if possible.

Exclusion Criteria:

• Subject in Treated Group:
• Subjects with a known presence of optic or peripheral nerve damage due to another illness, condition or medication.
• Subjects with a pre-existing or a diagnosis at time of screening visit of an ophthalmologic condition that would adversely affect the study testing protocol (e.g. dense cataracts, macular degeneration, retinitis pigmentosa).
• Subjects who are currently receiving or anticipated to receive another medication, antibiotic or other, that has known potential to produce ocular or neurologic toxicity or lactic acidosis.
• Subjects with a history of significant exposure, in the opinion of the investigator and with prior discussion with the medical monitor, to medications known to produce optic or peripheral neuropathy.
• Subjects with an active communicable disease (i.e., tuberculosis assessed as currently communicable) and subjects on active treatment for tuberculosis or other mycobacterial disease that include drugs that have known potential to produce ocular or neurologic toxicity.
• Subjects with severe liver disease or abnormal liver function test.
• Subjects in Control Group:
• Subjects must not currently be taking linezolid or have received it for more than 7 days at any time.
• Subjects with a known presence of optic or peripheral nerve damage due to another illness, condition or medication.
• Subjects with a pre-existing or a diagnosis at the screening visit of an ophthalmologic condition that would adversely affect the study testing protocol (e.g. dense cataracts, macular degeneration, retinitis pigmentosa).
• Subjects who are currently receiving another medication, antibiotic or other, that has known potential to produce ocular or neurologic toxicity or lactic acidosis.
• Subjects with a history of significant exposure, in the opinion of the investigator and with prior discussion with the medical monitor, to medications known to produce optic or peripheral neuropathy.
• Subjects with an active communicable disease (i.e., tuberculosis assessed as currently communicable) and subjects on active treatment for tuberculosis or other mycobacterial disease that include drugs that have known potential to produce ocular or neurologic toxicity.