JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS - Figure 1. Histology of soap-treated and control rat skin. (a) Tissue after 12 treatments with 8•o soap during 3 days. Arrow indicates infiltration of leucocytes into epidermis. (b) Control skin (12 treatments with water during 3 days). Stained with haematoxylin and eosin. x 408. Facing page 476

EFFECT OF SOAP UPON CERTAIN ASPECTS OF SKIN BIOCHEMISTRY 477 )ically applied soap (five treatments) Figure 2. The irritancy of sodium soaps of fatty acids. All individual fatty acid sodium soaps were 0.25 M, pH 9.8. The soap mixture was 8 •o, pH 9.8. The most irritant fatty acid soap was sodium laurate. The irritation response was so severe that in many regions of the skin studied histologically there was complete necrosis of the epidermis, thus, an evaluation of the irritation score could not be made. The sodium soap of decanoic acid also caused a severe irritation response, followed, in order of magnitude, by the complete soap solution used in the above studies, and sodium oleate. Soaps of the fatty acids C8:0, Cm0, Cm0 and C•s:0 gave irritation responses very close to those of the water-treated controls. It was concluded that the most irritant component of the complete soap formulation was sodium laurate. Effect of topical soap treatment on DNA synthesis in rat skin Twelve rats were divided into four groups of three animals. Group 1, the controls, were topically treated with water, receiving four treatments per day for a total of three days Group 2 were topically treated with 85/o soap solution, receiving a total of four treatments during 1 day Group 3 received eight soap treatments during 2 days and Group 4 received a total of 12 treatments during 3 days. After completion of respective topical treatments the animals of each group were killed and the degree of irritation of the treated areas examined histologically.