In this paper we address the theoretical problem of evaluating the quality option embedded in interest rate futures contracts. We use the martingale properties of the prices of interest-rate contingent claims under different probability measures in order to derive solutions for the value of futures and options on futures, accounting for the quality option and assuming a square-root model for the short rate. The futures pricing formula boils down to a simple linear combination of the futures prices of the zero-coupon bonds which constitute the deliverable bonds. A European call option on such a futures can be rewritten as an option on a single futures in which the strike price is 'curved', i.e. it is a decreasing function of the short rate.