We have measured the product of the ras oncogene, ras p21, in random un-timed urine samples using an immunoblotting method which relies upon enhanced chemiluminescence for visualisation of the nitrocellulose filter. Urine samples were analysed from groups of patients with prostate (n = 10) or bladder (n = 25) cancer and a control group (n = 30) with no apparent urological disease. The mean concentration of urinary ras p21 in the groups with either bladder or prostate cancer was not significantly higher than that of the control group. The most striking difference between the control and clinical groups was the presence of a previously un-reported ras p21 "doublet" in the electrophoretic patterns obtained from 20% of the bladder cancer group and 10% of the prostate cancer group. This doublet was not present in any of the control samples analysed. This doublet is strongly suggestive of a mutation within the ras oncogene.