Explantation and cultivation in vitro of 55 human biopsies of histologically normal urothelial tissue, and 63 biopsies from transitional cell carcinomas (TCC) revealed a primary success rate of about 75% and a secondary success rate of about 50% with no significant differences, between normal and TCC derived cultures. While only half of the subcultured biopsies of normal origin were able to grow for more than 4 passages all TCC derived cultures were able to do so. Based on morphology, life span, invasive growth into co-cultured fragments of embryonic chick heart, and tumorigenicity in nude mice the following transformation grades (TGr) were defined: Normal (TGr 0), non-malignant with prolonged, but finite life span (TGr I), premalignant with infinite life span but without tumorigenic and invasive potential (TGr II), malignant with infinite life span and tumorigenic and invasive properties (TGr III). Differentiation between TGr I - TGr III was possible by several other parameters, including cytological characteristics, molecular and immunological surface structure, relative oncogene expression, pyruvate kinase isoenzymes, DNA Repair, and growth pattern. However, in addition to these common characteristics, the individual cell lines of the various categories of transformation showed differences that allowed the identification of well defined cell lines and sublines.