The effects of an antitumor antibiotic neocarzinostatin (NCS) on the surface immunoglobulin central capping induced by anti-immunoglobulin M antibody on [human Burkitt''s lymphoma] Daudi cells and on the cell spreading of trypsinized [human cervical carcinoma] HeLa-S3 cells were examined. Pretreatment of Daudi cells and HeLa-S3 cells with NCS, 5-30 .mu.g/ml, for 4 h inhibited cap formation and cell spreading, respectively. There was a direct relationship between the dose and the degree of inhibition. DNA synthesis inhibitors such as bleomycin, mitomycin C and 1-.beta.-D-arabinofuranosylcytosine showed no inhibitory effect on cap formation or cell spreading. Known microtubule-acting agents such as colchicine and vinblastine sulfate completely inhibited capping and cell spreading at a dose of 10 .mu.g/ml. In view of the fact that 10 .mu.g/ NCS/ml also inhibits the formation of microtubular paracrystals induced by vinblastine sulfate in HeLa-S3 cells, and that other agents known to influence microtubule function such as local anesthetics and calcium ionophores inhibit both paracrystal formation and cap formation, these new observations support that NCS affects microtubular proteins transmembranously in vivo.