The waveguiding properties and amplified spontaneous emission (ASE) of a blend of light-emitting gain-conjugated polymers were investigated. ASE-induced line narrowing occurs for excitation fluences larger than $100\mu \mathrm{J}{\mathrm{cm}}^{-2}$ , with a maximum optical-gain coefficient of $8{\mathrm{cm}}^{-1}$ . Energy transfer between the host and guest polymers, significantly reducing the self-absorption, leads to a loss coefficient of the waveguide as low as $0.3{\mathrm{cm}}^{-1}$ , which is believed to be the lowest value reported for active organic gain slabs and a highly polarized emission, with a polarization contrast up to 0.65. These results indicate that gain-conjugated polymer blends are state-of-the-art organic materials for lasing devices.