We report the first field-effect transistors from blends of conjugated polymers and the determination of the field-effect mobility of electrons as a function of blend composition. It is found that the field-effect electron mobility (mu(e)) in a series of 12 binary blends of poly(benzobisimidazobenzophenanthroline) (BBL,mu(e) = 5 x 10(-4) cm(2)/Vs) and poly(p-phenylene-2,6-benzobisthiazole) (PBZT, mu(c) = 2 x 10(-7) cm(2)/Vs) has a novel "stair step" dependence on composition. The electron mobility is relatively high (5 x 10(-5) cm(2)/Vs) and constant over a wide blend composition range (x = 0.05-0.6, wt fraction of PBZT). At higher concentration of the lower-mobility component, the electron mobility falls exponentially with composition. The results suggest that thin film transistors with relatively high mobility may be realized in blends of conjugated polymers if one of the components has a sufficiently high mobility. The observed "stair step" electron mobility dependence on blend composition is not explained by current theories of hopping transport.