The morphological characteristics and the production of biochemical markers were determined for 8 human tumor cell lines grown in artificial capillary culture. Comparisons were made with nude mouse xenografts and conventional monolayer or suspension cultures. Capillary histologies reproduced the features of neoplastic differentiation and glandular formation exhibited by the original human tumors and xenografts. The concentrations of specific biochemical markers, such as carcinoembryonic antigen, aspartate aminotransferase, and immunoglobulin, were higher in the pericellular culture medium in capillary culture. The capillary environment influenced the expression of biochemical heterogeneity by the DLD-1 colon carcinoma cell line and its derivative clones. Spontaneous differentiation of K562 leukemia cells was increased in the capillary system. These results indicate that the artificial capillary is a useful and relevant system for the study of cultured human tumor cells.