The penetration of ceftazidime, a new broad-spectrum cephalosporin, into fluids from subcutaneous threads, suction blisters, and cantharidin blisters was studied in eight healthy male volunteers. A pharmacokinetic analysis showed fundamental differences between the models. The results obtained with the subcutaneous thread technique were similar to those of the peripheral compartment and were characteristic of a rapidly equilibrating compartment. The results obtained with the suction and cantharidin blister techniques were characteristic of slowly equilibrating compartments. We concluded that although one model will not accurately predict the penetration of an antibiotic in all clinical situations, each model will have its own particular application.