Expression of the glutathione S -transferase, GSTP1, is associated with phase 1 detoxification of the products of oxidative stress. Recently, GSTP1 expression has been implicated in the regulation of cell proliferation and apoptosis through direct interaction with the c-Jun N-terminal kinase, (JNK). GSTP1 is polymorphic and allelic variants have been associated with disease susceptibility and clinical outcome. However, the influence of GSTP1 alleles on proliferation and apoptosis has not been studied previously. To investigate this, we have examined the effects of inducible expression of wild-type GSTP1*A and mutant GSTP1*C haplotypes on cell proliferation and apoptosis in NIH3T3 fibroblasts. Cells expressing GSTP1*A displayed increased doubling times and a delayed G 1 –S phase transition compared with cells expressing GSTP1*C . Both GSTP1*A and GSTP1*C haplotypes protected cells from undergoing apoptosis when exposed to oxidative stress. However, analysis of JNK status revealed that only GSTP1*C expression led to a reduction in JNK activity compared with GSTP1*A -expressing cells and non-induced cells. We further examined the effect of GSTP1 alleles on colony-forming efficiency (CFE) in soft agar following exposure to oxidative stress and found that GSTP1*A -expressing clones had increased CFE compared with non-induced and GSTP1*C -expressing clones. Our data suggest that GSTP1 alleles have differential effects on proliferation and apoptosis; GSTP1*A reduces cellular proliferation and protects against apoptosis through a JNK-independent mechanism. In contrast, GSTP1*C does not influence cellular proliferation but protects cells from apoptosis through JNK-mediated mechanisms.