During growth of Agaricus bisporus fruit bodies the amount of active tyrosinase increased. The amount of active tyrosinase can be related to the degree of browning, as opposed to the fully activated tyrosinase level. Isoelectric focusing revealed that active and latent tyrosinase isoforms having different isoelectric points could be distinguished. The isoform composition was dependent on the tissue and developmental stage. In the later developmental stages new active isoforms appeared.