Building upon the seminal work of Easley, Kiefer, O’Hara and Paperman (1996), we develop a framework to investigate the relationship between the behavior of uninformed investors and the time-varying informed trading activities. We allow the arrival rates for uninformed traders to follow a Markov switching process where the transition probabilities depend on market fundamentals. Informed traders may match the level of the uninformed arrival rate with certain probability so as to make better use of the camouflage provided by the uninformed transactions. Our empirical estimation of NYSE stocks shows that the uninformed transition probabilities are indeed time-varying, so is the probability of information content. The estimated probability of information content predicts the opening, median and closing spreads. There is evidence that uninformed investors exhibit momentum chasing and “noise herding” behavior. There is also a positive “market spillover” effect in the uninformed trading activities. We find that the “clustering” of trading activities by uninformed and informed traders seem to be more likely on low volume days, and the uninformed trading activities are responsible for most of the stock trading volatilities.