Contamination and low viability of earthworm coelemocytes in tissue culture have delayed in vitro studies. Using penicillin, streptomycin, tetracycline and Amphotericin B, Lumbricus terrestis coelomocytes were maintained viable and uncontaminated for 10 days at 15degreesC in medium L-15 supplemented with 5 to 10% fetal bovine serum. The coelomocytes survived for at least 10 days with 85% viability as assessed by trypan blue exclusion assays and phagocytosis of heat-killed yeast. Studies on the thymidine uptake, however, were negative. With the involvement of coelomocytes in tissue graft rejection, in vitro techniques can now be applied to study their capacity in the immune response.