In the present paper

Mixed radioiodines were injected intraperitaneously into rats with dose of 148×10

4

74×10

4

37×10

4

Bq and null respectively in 4 groups to observe their carcinogenic effects.

The highest incidence of thyroid tumours in male rats treated with 74×10

4

Bq of radioiodines was 62.5% and the next was 54.6% in which treated with 37×10

4

Bq.

The highest in

cidence of tumours in female rats treated with 37×10

4

Bq of radioiodines was 57.1%. The majority of tumours was derived from thyroid and the next from mammary gland.

The thyroid tumours in both male and female rats were all originated from follicular epithelial of para-epithelial cells.

The main type of benign tumours was polymorphofollicular adenoma and the malignant one medullary carcinoma.

The carcinogenesis of mixed radioiodines was stronger than that from any single radioiodine which was a component of them

appearing the ratio of malignant to benign tumours being increased from 0.0 or 0.6 in the rats treated with only I-125

I-131

or I-132 up to 10.0.