<h3>Objective:</h3> To compare changes in plasma concentrations of Neurofilament light (NfL) and Glial Fibrillary Acidic Protein (GFAP) in individuals with relapsing multiple sclerosis (RMS) vs progressive multiple sclerosis (PMS), treated with anti-CD20 immunotherapy (rituximab or ocrelizumab) <h3>Background:</h3> Blood biomarkers, especially NfL, can augment clinical and radiographic data in MS treatment monitoring. Few studies have investigated longitudinal change in NfL or GFAP in PMS patients taking anti-CD20s or compared changes in these biomarkers between RMS and PMS. <h3>Design/Methods:</h3> Subjects were selected by MS diagnosis, anti-CD20 treatment for at least 6 months, and presence of multiple blood samples in our Center’s Biorepository. Demographic/clinical information was extracted by chart review. SIMOA plasma assays of NfL and GFAP were conducted at baseline and follow-up (between 3 and 12 months) on Quanterix SR-X. Biomarker concentrations were log transformed. Summary statistics and longitudinal regression analyses, correcting for age, were generated. <h3>Results:</h3> 37 subjects and 67 samples were analyzed (n=7 had no follow-up samples between 3 and 12 months). 64.9% were female. RMS (n=19) had mean(SD) age 38.7(9.2). PMS (n=18) had mean age 56.1(10.4). Age-adjusted mean concentrations of NfL and GFAP did not differ significantly between RMS and PMS at baseline. Geometric mean NfL declined between baseline (6.8, 95%CI 4.2–11.0) and follow-up (4.9, 95%CI 3.3–7.3) in RMS but stayed stable in PMS. Geometric mean GFAP increased between baseline (52.4, 95%CI 35.2–77.9) and follow-up (60.8, 95%CI 40.4–91.4) in PMS but stayed stable in RMS. The changes for both NfL and GFAP differed statistically significantly between MS types. <h3>Conclusions:</h3> With anti-CD20 treatment, plasma NfL levels decreased in RMS by 27.6% while staying stable in PMS; GFAP levels increased in PMS by 16% while remaining stable in RMS. Results correlate with clinical evidence showing greater benefit of anti-CD20 immunotherapy in RMS compared to PMS. Future studies will include more subjects, longer observation, and different immunotherapies. <b>Disclosure:</b> Dr. Gross has received personal compensation in the range of $5,000-$9,999 for serving as an Expert Witness for AP Expert Group. The institution of Stefan Sillau has received research support from Alzheimer’s Association. The institution of Stefan Sillau has received research support from Hewitt Family Foundation; State of Colorado. The institution of Stefan Sillau has received research support from PCORI. The institution of Stefan Sillau has received research support from NINR. The institution of Stefan Sillau has received research support from Michael J. Fox Foundation. The institution of Stefan Sillau has received research support from Department of Defense. The institution of Stefan Sillau has received research support from Colorado Department of Public Health and Environment. The institution of Stefan Sillau has received research support from Benign Essential Blepharospasm Research Foundation. Stefan Sillau has a non-compensated relationship as a Statistician with Novartis that is relevant to AAN interests or activities. Stefan Sillau has a non-compensated relationship as a Statistician with Biogen that is relevant to AAN interests or activities. Sean Selva has nothing to disclose. Mrs. Ritchie has nothing to disclose.