Both retinol serums provide anti-ageing benefits and are a powerful addition to your skincare routine. The main difference between these face serum is the percentage of retinol they contain. Kiehl's Skin-Renewing Micro-Dose Retinol Serum uses a mico-dose of retinol that allows you to introduce the ingredient to skin, and is the best Kiehl's retinol serum for beginners and those with sensitive skin. The Fast-Release Pure Retinol Night Serum uses Kiehl's highest concentration of retinol yet, helping to visibly reduce the appearence of fine lines and wrinkles quickly by penetrating 15 surface layers of skin. Learn more about Kiehl's retinol percentages with our 'How much retinol to use' guide.