Five hays of different quality were used as the source of both inoculum and substrate for in vitro digestion trials. They contained 18.0, 16.2, 13.9, 11.5, and 6.7% protein and were designated alfalfa 1, 2, 3, grass hay, and native prairie hay, respectively. Two fistulated cows were used as donor animals. Digestible cellulose was used as the criterion for evaluating the inocula. There was no significant difference between the 3 alfalfa hays as the source of inoculum. However, digestion by alfalfa 1 and alfalfa 3 inocula was significantly (P < .05) greater than that from grass or native hay, and inoculum from the grass hay was superior (P< .05) to that from the native hay. As a source of inoculum the [center dot] native prairie hay was more (P < .05) variable than the other hays. There was no significant difference in the digestibility of the cellulose when the inoculum and substrate were from the same hay or from different hays. A highly significant difference existed between the activity of the inocula from the 2 donor cows.