We report a study of light amplification in dilute solutions of the semiconducting polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). The operating characteristics of the F8BT solution amplifier are investigated in detail, and we show that it provides a large optical gain (>40dB∕cm) across a wide (>48nm) spectral bandwidth in the green. The saturation characteristics of the amplifier are found to be in excellent agreement with theoretical predictions, allowing an estimate of the F8BT stimulated emission cross-section, σ. We find σ∼1.7×10−16cm2, comparable with the reported values for established laser dyes and other high-gain conjugated polymers.