Tyrosine phosphorylation has emerged as an important mechanism in the regulation of enzyme function. In this paper, we describe a mutant of PKCdelta altered at a single tyrosine residue which has the opposite effect compared with wild-type PKCdelta on the growth characteristics of NIH 3T3 cells. Overexpression of wild-type PKCdelta results in a decreased growth rate and a lower cell density at confluency. On the other hand, overexpression of PKCdelta with a mutation from tyrosine to phenylalanine at position 155 results in a significantly higher rate of growth and a higher density at confluency compared with vector controls. Moreover, these cells are able to grow in soft agar and to form tumors in nude mice. In contrast to kinase negative PKC constructs, this mutant maintains in vitro kinase activity and shows a subcellular localization and a translocation pattern that are similar to those of the wild-type PKCdelta. Whether the altered biological effect is due to the missing phosphorylation on tyrosine or the mutation from tyrosine to phenylalanine per se remains under investigation.