The aim of the research The aim of the research is to examine the potential cytotoxic effects of extracts from the Brassica genus of plants on human urinary bladder carcinoma cells (T24) and lung carcinoma cells (A549). The assumption is that after the exposure of carcinoma cells to extracts their number will be reduced in comparison to the control group. Materials and methods MTT assay was used to establish cytotoxicity levels on urinary bladder cancer cell lines. The comparison between the absorbance of created formazan in cells treated with extracts and those untreated (the control group) established the survival rates of carcinoma cells. Cytotoxicity levels were established after the periods of 4, 24, 48 and 72 hours. Results The results are graphically presented in relation to incubation times and the percentages of metabolically active cells. In urinary bladder carcinoma cells T24 all extracts show better effects in higher concentrations of 50 μg/mL and 100 μg/mL, with different incubation times. The most prominent effect on the reduction of metabolically active cells is achieved by microvawe extract of Brassica oleracea L. var. capitata f. rubra at a concentration of 100 μg/mL after 72-hour incubation time. In lung carcinoma cells A549 most extracts have the strongest effect at higher concentrations of 50 μg/mL and 100 μg/mL, with different incubation times. The most significant effect on the reduction of metabolically active cells is achieved by microvawe extract of Brassica oleracea L. var. capitata f. rubra at a concentration of 100 μg/mL after 72-hour incubation time. Conclusions In vitro exposure of urinary bladder carcinoma cells (T24) and lung carcinoma cells (A549) to extracts leads to reduced cell survival rate. Cytotoxic effects depend on incubation time and concentration. Herbal extracts effect is not always proportional to the increase in concentration and incubation time and in some cases cell recovery occurs. The cytotoxic effect of extracts is confirmed, which was the hypothesis of the research, and the next step is the validation of these effects by in vivo studies on the models of animal urinary bladder and lung cancers.