Title: Model Validation Using Mutated Training Labels: An Exploratory Study. Abstract: For out-of-sample validation, the sample set may be too small to be representative of the data distribution; the accuracy can have a large variance across different runs; excessive reuse of a fixed set of samples can lead to overfitting even if the samples are held out and not used in the training process. This paper introduces an exploratory study on Mutation Validation (MV), a model validation method using mutated training labels for supervised learning. MV mutates training data labels, retrains the model against the mutated data, then uses the metamorphic relation capturing the consequent training performance changes to assess model fit. It uses neither validation nor test set. The intuition underpinning MV is that overfitted models tend to fit noise in the training data. We explore 8 different learning algorithms, 18 datasets, and 5 types of hyperparameter tuning tasks. Our results demonstrate that MV is accurate in model selection: the model recommendation hit rate is 92% for MV and less than 60% for out-of-sample validation. MV also provides more stable hyperparameter tuning results than out-of-sample validation across different runs.