many different agents , including mineral oil and silicone , have the capacity to act as immunological adjuvants , i.e. , they can contribute to the activation of the immune system . 
 some adjuvants , including mineral oil , are known to induce arthritis in certain strains of rats after intradermal injection or percutaneous application . 
 the aim of this study was to determine if common commercial cosmetic products containing mineral oil could induce arthritis in the highly susceptible da ( dark agouti ) rat . 
 intradermal injection of five out of eight assayed cosmetic products without further additives resulted in arthritis with synovitis . 
 one of the products induced a very aggressive arthritis , which had declined after 5 - 9 weeks . when this product was also assayed for arthritogenicity upon percutaneous administration 
 , it induced a mild and transient arthritis in 5 out of 10 da rats , whereas control animals showed no clinical signs of joint involvement . 
 no arthritic reaction was seen in rats after peroral feeding with the most arthritogenic product or by intravaginal application of freund 's adjuvants . 
 silicone gel implants in da rats did not cause arthritis . 
 we conclude that mineral oils included in common commercially available products retain their adjuvant properties and are arthritogenic in the presently investigated arthritis - prone rat strain . 
 there is yet no evidence that mineral oils present in cosmetics may contribute to arthritis in humans , but we suggest that this question should be subject to further investigation.imagesfigure 1figure 2figure 3figure 4figure 5figure 6