To the Editor.— Although Algra et al were unable to detect clindamycin in the serum following topical application of a hydroalcoholic solution of clindamycin hydrochloride (Archives 113:1390-1391, 1977), The Upjohn Company has observed clindamycin concentrations of up to 0.7 μg/ml in the urine of four of nine subjects who were receiving topical applications of 20 mg of clindamycin daily for one to seven weeks, which indicates that some systemic absorption does occur. In addition, The Upjohn Company has received three reports of diarrhea associated with topical clindamycin therapy for acne. Proctoscopy in one of these cases showed markedly inflamed mucosa, which resolved following cessation of clindamycin. In another patient, the diarrhea cleared with discontinuation of topical clindamycin and reappeared following resumption of this topical medication. In the third case, diarrhea occurred following topical application of clindamycin in a patient who had previously experienced diarrhea with oral clindamycin (H. W. Woodward,