Studies in recent years have indicated that secretion of certain lysosomal hydrolases can be enhanced under various conditions. One such protein, the major excreted protein (MEP) of Kirsten virus-transformed NIH 3T3 (KNIH) fibroblasts, is a lysosomal cysteine protease whose synthesis and secretion are affected by viral transformation and growth factors. We have been studying the synthesis and transport of MEP in order to understand better the mechanisms responsible for regulation of lysosomal enzyme secretion. Synthesis of MEP in KNIH cells was found to be 25-fold greater than that in untransformed NIH cells, and 94% of the MEP made was secreted. This was in contrast to NIH cells which secreted only 11% of the newly synthesized MEP. The high level of secretion by the transformed cells was relatively specific in that most other lysosomal enzymes were retained. MEP isolated from both NIH and KNIH cells exhibited a low intrinsic affinity for the mannose-6-phosphate receptor which was at least 10-fold lower than that of other lysosomal enzymes. On the basis of these results, we suggest that both the high level of MEP synthesis and the intrinsic low affinity of MEP for the receptor are responsible for the specific increase in MEP secretion by transformed cells.