The existence of energy-dependent copper influx was demonstrated in protoplasts of Penicillium ochro-chloron. Protoplasts and mycelium were tolerant of copper over the pH range 3·0 to 5·5 but sensitive at pH 6·0. Uptake of copper was approximately ten times greater at pH 6·0 than in the lower range. At pH 3·0, influx showed saturation kinetics with a half-maximal influx at an external Cu2+ concentration of 390 μ;M and a maximum influx rate of 22 nmol h−1 (107 cells)−1. Saturation kinetics were not observed at pH 6·0.