The cytoplasmic proteins of Neurospora crassa 3a6A (wild strain) depending on the time of mycelial growth-are characterized by means of agar gel electrophoresis, acetate-foil electrophoresis, immunelectrophoresis, agar gel diffusion technique, labelling with 59Fe and 5sC0 and by detection of the lactate dehydrogenase-isoenzymes. The protein composition of the mycelium changes with age. During mycelial growth the fraction of watersoluble proteins increases. Simultaneously, quantitative differences are evident in the water-soluble protein fraction. The patterns of immunelectrophoretical precipitation depend on mycelial age. 2 Fe-labelled precipitation arcs are detectable. Also the stability of the Co-proteides depends on the age of the mould. The distribution of the lactate dehydrogenase-isoenzymes varies during mycelial growth.