p53 mutations in human tumors are often associated with overexpression of p53, and immunohistochemical detection of p53 has frequently been chosen as a simpler method than genetic analysis to access p53 mutations. In this study, we analyzed the p53 gene by single-strand conformational polymorphism (SSCP) and DNA sequencing, and correlated findings to Ab staining results. In a series of 58 squamous cell carcinoma, 15 showed mutations in exons 5, 6, 7, 8 and 9 by SSCP. Of these 15 cases, 11 were positive by antibody staining, and DNA sequencing showed missense mutations but no frameshift or nonsense mutations. In contrast, the antibody-negative cases had frameshift or nonsense mutations, but no missense mutations. SSCP analysis of these 4 cases showed mutations in exon 6 (2 cases), exon 7 (1), and exon 8 (1), respectively. In case 1, sequencing data revealed a single-base addition in exon 6, leading to a truncated gene product of 207 amino acids (aa), in contrast to 393 aa in wild-type p53. Similar frameshift mutations were shown in case 2 and case 3. Case 4, instead of a frameshift mutation, carried a nonsense mutation, and a truncated peptide of 235 aa. All these mutations thus shared the feature of producing truncated p53 products nonreactive with antibodies. We conclude that frameshift mutations as well as nonsense mutations can lead to altered p53 undetectable by available monoclonal antibodies. Our finding indicates that the absence of Ab reactivity does not rule out genetic alterations of the p53 gene in human tumors.