Sexual development in Aspergillus nidulans is a morphogenetic differentiation process triggered by internal and environmental signals. As a first step in analyzing the developmental pathway at the molecular level, laccase II (EC 1.10.3.2), which is specifically expressed in early stages of fruitbodies, was isolated. The enzyme was purified to apparent homogeneity from a mutant strain (SMS1) in which the sexual cycle dominates and the number of cleistothecia is increased tenfold. Laccase II was enriched 560-fold to a specific activity of 892 U (mg protein)-1. The apparent molecular mass was determined to be 80 kDa under denaturing conditions and to be 100-120 kDa under native conditions. The internal peptide sequences gained from the protein will allow the isolation of the corresponding gene as a first step in determining the key regulators of sexual development.