Uptake of [3H]colchicine (2.5 ng/ml) by human lymphocytes in culture was slow in the length of time to reach steady state (> 48 hr) and was limited in the maximal intracellular colchicine amount (1-2% of total extracellular colchicine). Efflux of intracellular colchicine was investigated 40 hr after colchicine cell exposure by using either washing of the extracellular medium or adding different colchicine-specific Fab fragments:colchicine dose molar ratios of 0.5, 1 and 5. Except for the 0.5 dose molar ratio, the kinetics of [3H]colchicine efflux from lymphocytes induced by extracellular specific Fab fragments were similar to those obtained by washing and were characterized by a first-order decline with half-lives ranging from 15.5 to 16.4 hr. These half-lives were in the same range as those characterizing the dissociation of colchicine from the intracellular tubulin receptor. Our data demonstrate that a tightly bound intracellular toxin may be extracted by antibody with high affinity for the toxin present in the extracellular space at a rate depending on the rate of dissociation of the toxin from its receptor.