For investigation of the relation of cell cycle regulation with tumorigenesis in cats, we cloned feline p21WAF1 and p27Kip1 cDNAs and searched for their aberration in feline spontaneous leukemias and lymphomas. The feline p21WAF1 cDNA (pCFW.31) clone obtained from the PCR amplified product appeared to cover approximately 75% of the open reading frame, and showed 81.6% and 76.8% sequence similarities with those of human and mouse counterparts, respectively. The pHFK.5 clone isolated by plaque hybridization contained the whole open reading frame of cat p27Kip1 cDNA encoding 198 amino acids, showing 93.4% and 90.4% sequence similarities with those of human and mouse counterparts, respectively. Southern-blot analyses using these clones as probes did not show any deletion or rearrangement of both the p21WAF1 and p27Kip1 genes in 19 feline spontaneous cases of leukemias and lymphomas examined. RT-PCR/SSCP (single strand conformation polymorphism) analysis of p27Kip1 cDNA indicated that there was no mutation resulting in amino-acid substitution in 10 feline leukemia and lymphoma cases.