The objective of this study was to revaluate the neutral detergent cellulase (NDC) method to predict in vivo organic matter digestibility (OMD). A dataset comprising of 87 perennial ryegrass (Lolium perenne L.) samples were generated from five studies measuring in vivo digestibility using sheep. In vivo OMD data ranged from 640 g kg-1 to 880 g kg-1. All sheep were offered fresh perennial ryegrass for ad libitum consumption. Grass samples were freeze dried at -55 °C for 72 h. After freeze drying, grass samples were milled through a 1 mm screen. All samples were analysed in triplicate for NDC to determine organic matter solubility (OMS), using the procedure outlined by Morgan et al. (1989). The NDC OMS results were regressed against in vivo OMD. The derived equation was: in vivo OMD = 0.540 (NDC OMS) + 0.348. The R2 was 0.745 (r.s.d = 0.028, P < 0.001) indicating a significant positive correlation between in vivo OMD and NDC OMS. The results from this study reaffirm the use of NDC method to accurately predict in vivo OMD.