In a recent outbreak of hand eczema amongst cleaning personnel after the introduction of a new, lemon‐scented detergent, it was noted that the patients complained of a burning, stinging sensation when their hands were submerged in hot detergent solutions. Since routine patch testing with the Standard Series and perfume components was of no help in pinpointing the responsible agent, testing with selected perfume components was repeated at higher temperatures. Identical tests were placed on both forearms for 20 min, one arm being exposed to 43°C. the other to 23–25 °C. Little or no reaction was seen on the “cold” arm, whereas the lemon perfume component Citral proved to be a strong primary irritant at higher temperatures.Histological examination of the test sites showed the reaction to be of a toxic (primary irritant) nature. Surprisingly, the toxic character could still be recognized in biopsies taken as late as 48 h after exposure.It is suggested that:1. Heat plays an important part in the outbreak of primary irritant (toxic) dermatitis and can be used as an accelerating factor when testing for primary irritants.2. It is important to be sure that detergents and detergent perfumes do not contain substances which act as irritants at the temperatures at which they are habitually used (45–50°C).3. We probably ought to use lukewarm rather than hot water for manual dishwashing and cleaning whenever it