Fifty-three solid and 33 fine-needle aspirate (FNA) samples (20 paired) of human breast carcinomas were examined by flow cytometry. Experiments were conducted to assess whether FNA samples were phenotypically representative of the solid tumour. Quantification of oestrogen receptor (ER), epidermal growth factor receptor (EGFR), c-erbB-2 receptor levels and ploidy were examined on the total and cytokeratin-positive cell populations. The absolute number of molecules of cytokeratin per cell expressed on the FNA (n = 33) and solid tumour (n = 53) samples showed no significant difference, but, on a proportional basis, there was a significant difference between the two samples (P = 0.004), with lower expression exhibited by the FNAs. Examination of paired data showed no significant difference in the percentage of cytokeratin-positive cells (P = 0.51) or in the number of cytokeratin molecules expressed (P = 0.25). While the correlation for ER expression between paired tumour and FNA samples in the absence of cytokeratin gating was P = 0.06, r2 = 0.18, clear correlation was shown when a cytokeratin gate was used (P = 0.005, r2 = 0.4). Repeating this experiment for EGFR, it was found that no correlation was seen between FNA and solid tumour (P = 0.2, r2 = 0.14) in ungated populations, but use of the cytokeratin gate improved the correlation (P = 0.05, r2 = 0.3). A similar finding was seen with c-erbB-2 expression (P = 0.2, r2 = 0.1) without cytokeratin gating and when it was employed (P = 0.05, r2 = 0.4). Ploidy data showed concordance in 18/20 cases. Three cases of aneuploidy were missed by FNA, and this was because of an insufficient number of cells for analysis. The presented data suggest that FNAs are representative of solid tumours and may be useful for measuring receptor levels on clinical material when cytokeratin gating is used. However, observation by light microscopy is still necessary to confirm the presence of tumour cells in FNAs subjected to flow cytometry.