The objective was to determine the effects of the Ankom F57 filter bags (F) on in vitro undigested NDF (uNDF) and gas production (IVGP). Six grass (GR), alfalfa (AL), and corn silage (CS) materials were selected for each type. Standard deviation of aNDFom indicated diversity: 58.4 ± 20.7, 40.0 ± 7.45, and 40.7 ± 12.94% DM, respectively, for GR, AL and CS; starch for CS was 28.4 ± 16.4% DM. Samples were ground to 1mm and 0.5g was weighed as a loose powder (L) into 250 mL fermentation bottles or into F bags, which were weighed to prevent floating in bottles. Blended ruminal fluid from 3 steers fed a high-NDF diet (30% DM) was used in the Goering and Van Soest (1970) in vitro method to measure IVGP using the Ankom RF system. For each material, 3 IVGP bottles were removed to measure uNDF at 12h (uNDF12h) and 2 were used for uNDF120h. For L, residues were extracted in ND using Gooch crucibles. For F, the bags were extracted using an Ankom Fiber Extractor and fermentation liquid was extracted in ND using Gooch crucibles to determine washout. There were 6 IVGP runs, but 2 runs were truncated due to computer malfunctions. Results were analyzed using MiniTab software with material within type as a random effect to account for type and IVGP run being confounded. Statistical models included type, material(type), treatment (L or F), and type × treatment interaction. Gas produced by blanks were evaluated at 12h and peak IVGP and F was larger than L for both (P < 0.005) and runs differed (P < 0.001). Differences in IVGP were evaluated at 3, 4.5, 6, 9, 12, 18, 24, 36, 48, 72 and 96h. By 3h, IVGP of F was less than L (P < 0.05). Maximum F < L difference in IVGP was largest at 24h (P < 0.001). There was a trend for IVGP of CS to be greater than AL or GR (P < 0.10) at 18h. Precision of repeated measurements for uNDF12h were 1.05 and 0.88 for F and L, respectively, and for uNDF120h were 1.26 and 0.36, respectively. The uNDF12h of F was greater than L (P < 0.01) for CS (37.0 > 30.0% DM) and AL (38.9 > 34.4% DM), but not for GR. For uNDF120h, F was greater than L (P < 0.05) only for CS (15.2 > 12.9% of DM). Results indicate that F57 bags inhibit IVGP and increase uNDF for some materials.