The arrival of Herceptin (Trastuzumab), an antibody against the HER-2 oncogene found in a proportion of breast carcinomas and other carcinomas, has emphasised the need for a standardised technique for demonstrating overexpression of HER-2. We compared the Dako A485 antibody and Dako HercepTest kit (HT) on a series of 122 breast carcinomas. Fluorescence in situ hybridisation (FISH) (Vysis) was performed on all cases with positive or equivocal immunohistochemical results. The Dako A485 showed HER-2 overexpression in 53% of carcinomas, while the HT showed 21% positive (HT 2+ 8%, HT 3+ 13%) and 79% negative (HT 0 67%, HT 1+ 12%). FISH for HER-2 gene amplification on all the HT 1+ and HT 2+ cases was negative, whereas FISH analysis of all HT 3+ cases was positive, with the exception of one case which could not be analysed for technical reasons. When histological subtype was analysed, only grade 3 infiltrating duct carcinomas were FISH-positive, suggesting that histological grading and subtyping may be able to triage carcinomas suitable for HER-2 testing. We suggest that the HT or a similar standardised immunohistochemical study for HER-2 can be used to screen breast carcinomas. We then recommend FISH where the carcinoma is HT 2+. FISH may also be appropriate in high grade, HT 1+ carcinomas where there are doubts regarding optimal tissue fixation or block storage conditions.