A two-stage in vitro assay procedure was developed for predicting the efficacy of microbial enzyme sources in rye-based diets for broiler chicks. The procedure uses the complete diet as substrate to predict the in vivo intestinal viscosity and final weight of birds fed such diets. The optimum conditions for maximizing in vitro viscosity of a rye-based diet consisted of digesting .6 g of diet (ground through a 1-mm screen) in .9 mL of .1 N HCl containing 2,000 U pepsin/mL for 45 min at 40 C. At the end of this incubation phase, .3 mL 1 M NaHCO3 containing 8 mg pancreatin/mL (8 x USP) was added and the tube sealed and incubated for 2 h with intermittent vortexing. The digesta was then centrifuged (12,700 x g) and the supernatant collected for viscosity analysis using a digital viscometer. This assay was used for a rye-based diet containing either 0, .1, .2, .4, .8, or 1.6% of a xylanase source (experimental Trichoderma longibrachiatum product). The results were compared to weight and intestinal viscosity proximal and distal to Meckel's diverticulum of broilers (19 days of age) that had been fed these diets. The in vitro assay accurately predicted the in vivo intestinal viscosity (proximal r2 = .758, P < .0001, distal r2 = .667, P < .0001) and final weight of these birds (r2 = .660, P < .0001). The data suggest that the in vitro assay is a reliable assay for assessing the growth-promoting ability of an enzyme preparation in chicks fed rye-based diets.