1 The clindamycin bioactivity was measured during the dosage interval in the plasma of women with puerperal infections and in their breast milk. 2 There was a marked interindividual variation in the peak levels. The clindamycin bioactivity in the milk ranged from 1/10 to several times the corresponding bioactivity in the plasma that was collected at the same time. 3 The concentration of clindamycin in the milk (bioactivity) at the end of the dosage interval correlated with the area under the plasma concentration v time curve. 4 Clindamycin is thus transferred into human breast milk. Although the actual amounts secreted are small, the wellknown side effects and the lack of knowledge about the disposition and effects of clindamycin in newborn infants are strong arguments against nursing during treatment with this drug.