Immunocytochemical localisation of mutant p53 in breast tumours serves as a potential prognostic molecular marker. In order to study the expression of p53 protein in breast cancer which constitutes the second most common malignancy in the South Indian female population, MAb CIBCVMC12 has been generated against human p53 protein isolated and purified from bacterial cell lysate of E.coli carrying the plasmid T 7-7 Hup53 grown in Luria broth to induce the expression of p53. The positive clones selected by ELISA were found to exhibit strong staining of nuclear p53 in both fresh and archival paraffin embedded breast tumour tissue sections. Commercial MAb D 07 against p53 was used as control. In immunoprecipitation, this MAb of IgG2b isotype was found to bind specifically to a protein of 53 kD. Immuno cyto chemical assay of normal, benign and malignant breast tissues of different histological types revealed that the majority of tumour cells were strong positive in the case of infiltrating ductal and lobular carcinomas, the staining being less intense for in situ carcinoma. The test for normal and benign tissues was negative. The staining patterns were comparable with those of control antibody. These results suggest that the MAb generated is specific to p53. The p53 protein expression was compared with the estrogen receptor (ER) status for 50 breast tumours which revealed that 38% of these were p53 positive and of these two were ER+. Among the p53 negative tumours, 48% were found to be ER+. A comparison of the p53 expression for 100 breast cancer patients indicated that 57% of the tumours were p53 negative and these patients had a longer overall 5 year survival rate and recurrent free interval which is statistically very significant. These results might suggest that p53 positive tumours are more aggressive biologically with poor prognosis.