Normal tissues contain only trace amounts of monoacetylated polyamines. N1-Acetylspermidine is present in high concentrations in mouse liver cells damaged by hepatotoxins and is also found in specialised cells of the hamster epididymis. In the present study human breast cancers were analysed for the presence of monoacetylated polyamines because N1-acetylspermidine is selectively elevated in human colorectal cancers. Free and monoacetylated polyamines (N1-acetylspermidine, N8-acetylspermidine and N1-acetylspermine), measured by high-performance liquid chromatography, were expressed as nmol/g wet wt of breast cancers (n = 54) or normal breast tissue (n = 15). Putrescine and monoacetylated polyamines were absent from normal breast tissue. Mean total content of monoacetylated polyamines in breast cancers 14.9 +/- 5.3 (S.E.) exceeded the mean total content of free polyamines (8.3 +/- 1.0) in normal breast tissue. Detectable levels of at least two of the monoacetylated polyamines were found in all breast cancers: N1-acetylspermidine was present in 51 (13.1 +/- 6.3), N8-acetylspermidine in 32 (0.6 +/- 0.1) and N1-acetylspermine in 28 tumours (1.2 +/- 0.3). There was no correlation between monoacetylated polyamine content of breast cancers and factors known to affect survival, i.e. tumour size, histological grade, oestrogen receptor status and node status. Monoacetylated polyamines are present in human breast cancers but not in normal breast tissue, implying that polyamine catabolism in breast cancers differs from that in normal breast tissue.