Protein-protein interactions (PPIs) play an essential role in various biological processes. A range of computational methods have been proposed to predict PPIs from protein sequences. Among these, homology-based methods and machine-learning methods have been widely used. However, to the best of our knowledge, these two methods have not been compared using the same dataset. Thus in this study, we have developed both homology-based and machine-learning methods to predict PPIs from amino-acid sequences and compared the prediction results. In the homology-based method, BLASTP search was used to identify sequence homology. Regarding the machine-learning methods, two popular methods, support vector machine and random forest, as well as six different protein features, were employed to build classifiers. We collected the PPI pairs with high-confidence scores from HitPredict4 to build the positive dataset and we built the negative dataset from the Negatome 2.0 database, in which non-interacting pairs were verified by experiments and 3D structure analysis. Our results show that machine-learning methods achieved better performance than homology-based method but there are many PPIs that are predicted only by the homology-based method. The integration of the two methods is expected to enhance the performance.KEYWORDS: protein-protein interactions, protein-protein interaction site prediction, support vector machine, machine learning, homology search.