Using polymerase chain reaction amplification and oligonucleotide probing, the activation of ras oncogenes in 24 benign and 20 malignant human thyroid neoplasms was examined. The frequency of ras oncogene activation was similar at all stages of tumorigenesis in this system, being found in 33% of adenomas overall (50% of microfollicular tumours), 53% of differentiated follicular carcinomas and 60% of undifferentiated carcinomas. This supports the contention that mutation of these oncogenes occurs at an early step in tumorigenesis. The predominant amino acid substitution in the differentiated tumours was glutamine to arginine at position 61 of Ha-ras or N-ras, but this mutation was not found in any of the undifferentiated tumours. It was noted that while transition mutations predominated in differentiated tumours (both benign and malignant), transversions were more common in the undifferentiated tumours.