While p14(ARF) suppression of tumorigenesis in a p53-dependent manner is well studied, the mechanism by which p14(ARF) inhibits tumorigenesis independently of p53 remains elusive. A variety of factors have been reported to play a role in this latter process. We report here that p14(ARF) displays different effects on the anchorage-dependent and -independent growth of p53-null/Mdm2 wild type cells. p14(ARF) blocks both the anchorage-dependent and-independent (soft agar) proliferation of 293T and p53(-/-) HCT116, but not p53-null H1299 lung carcinoma cells. While p14(ARF) had no effect on the anchorage-dependent proliferation of p53(-/-) MEFs and Ras12V-transformed p53(-/-) MEFs, it inhibited the growth of Ras12V-transformed p53(-/-) MEFs in soft agar. Furthermore, ectopic expression of p14(ARF) did not lead to degradation of the E2F1 protein and did not result in the reduction of E2F1 activity detected by two E2F1 responsible promoters, Apaf1 and p14(ARF) promoter, in 293T, p53(-/-) HCT116, and H1299 cells. This is consistent with our observations that p14(ARF) did not result in G1 arrest, but induced apoptosis via Bax up-regulation. Taken together, our data demonstrate that the response of p53-null cells to ARF is cell type dependent and involves factors other than Mdm2 and E2F1.