Sphingosine and sphinganine, free sphingolipids of the stratum corneum, are, in vitro, strongly inhibitory for both bacteria and fungi. Whether or not they are suitable, indeed active, in vivo was examined: (i) on human volunteers, first as a preventative antiseptic against subsequently applied Staphylococcus aureus and Candida albicans, and second as a restorative antiseptic against the previously expanded normal skin flora; and (ii) on guinea-pigs as therapy for experimental C. albicans and Trichophyton mentagrophytes infections. In the antiseptic studies, which involved 200 micrograms/cm2 of sphinganine in ethanol (50 microliters of a 1.6% solution), up to three-log reductions in the population of target micro-organisms were obtained, compared with vehicle and untreated controls (P < 0.001). The daily application of sphingosine as 1.5% ethanol-petrolatum ointment was able to diminish inflammation slightly in dermatophyte-infected guinea-pigs (P = 0.02-0.05), although the animals remained culture positive over the 3-week sampling period. The candida infections, treated daily with 1.5% sphinganine in ethanol, showed no improvement in inflammation compared with controls, except for 2 days of the 2-week observation period (P = 0.01-0.03); however, by the fourth day of therapy the yeast was eliminated in 75% of animals. No gross toxicity was observed among animals or human volunteers. These experiments further support simple sphingolipids as important antimicrobial agents of the cutaneous barrier and point toward a new biochemical approach in treating infectious disease.