Seven proprietary repellent formulations (3 hydro-alcoholic spray solutions and 4 skin lotions) with active ingredient IR3,535 (ethyl butylacetylaminopropionate, EBAAP) or Picaridin (hydroxyethyl isobutyl piperidine carboxylate, KBR 3,023, Bayrepel) were tested in a field study on 10 test persons over a period of 10 h for their efficacy at preventing bites. The tests were conducted in Belo Horizonte, Brazil on field populations of the yellow fever mosquito Aedes aegypti. The concentration of the active substances ranged from 10% to 20%. All the tested samples provided lasting protection (time to first bite) over several hours: ranging from 5 h 20 min to 6 h 50 min with a mean of approximately 6 h. The longest protection until the second bite (=first confirmation bite) was approximately 7 h 40 min, whereas the shortest protection was 6 h 50 min. The longest protection until the third bite (=second confirmation bite) was 8 h 35 min, whereas the shortest protection was 7 h 40 min. In the control tests in which none of the samples were applied, the mean times until the first, second and third bites were 26, 46 and 59 min, respectively. The basis for this field study was provided by two American guidelines, which have the greatest international acceptance. The first is a draft guideline from the Environmental Protection Agency (EPA (United States Environmental Protection Agency), Product performance test guidelines. OPPTS 810.3700. Insect repellents for human skin and outdoor premises. Public Draft, 1999) and the second is a standard from the American Society for Testing and Materials (ASTM (American Society for Testing and Materials International), E 939-94 (reapproved 2,000): standard test method of field testing topical applications of compounds as repellents for medically important and pest arthropods (including insects, ticks, and mites): I. Mosquitoes, 2,000). Both guidelines recommend measuring the duration of protection until the first and second bites and also determining the relative protection efficacy in terms of a 95% protection level. The ASTM standard permits different repellents to be applied, whereas the EPA guidelines only permit the use of a single repellent (in different concentrations) on the extremities (forearms or lower leg). In the study presented here, to exclude any possibility of different repellents or concentrations of a single repellent having a reciprocal effect on each other, each test person had repellent samples applied to only one of their forearms. The other forearm was used as a control for making comparative checks every hour and for determining the biting pressure. There was no significant difference in protection times between the two active substances.