The percutaneous absorption of clindamycin was studied in healthy male volunteers, comparing two investigative clindamycin (% w/v)/tretinoin (0.025% w/v) gels, containing clindamycin phosphate ester and clindamycin HCl, respectively, relative to a clindamycin phosphate lotion (1% clindamycin; Dalacin T). Formulations were applied daily for 5 days on the face, according to a balanced complete block design. Redness of the skin was scored visually, and blood and urine were collected. Clindamycin plasma levels did not exceed the limit of quantification (5 ng mL(-1)) with the clindamycin phosphate formulations, but one volunteer who received the clindamycin HCl/tretinoin gel showed plasma levels of up to 13 ng mL(-1). Clindamycin urinary excretion for 12 h after application of the clindamycin phosphate/tretinoin gel was comparable to the values of the reference lotion, whereas the clindamycin HCl/tretinoin gel gave significantly higher values. Erythema appeared to be associated with increased urinary excretion. The formulations were tolerated well. In a separate clinical pilot study in acne patients, the transdermal uptake of tretinoin and clindamycin from the clindamycin phosphate/tretinoin gel was monitored. Plasma samples were collected after 4 and 12 weeks of daily treatment. None of the study plasma samples contained measurable tretinoin levels. Clindamycin levels were not quantifiable in the majority (87%) of samples, the highest plasma level was 11 ng mL(-1). The chemical form of clindamycin proved to modulate skin irritation and percutaneous uptake of clindamycin from a gel formulation in healthy subjects. There was no indications for a notable transdermal uptake of tretinoin during daily application of the gel in patients, nor for an enhancing effect of tretinoin on clindamycin uptake.