Epidemiological studies have shown a relationship between diets rich in tomato and/or lycopene and a reduction in cancer rates. Several studies reported reductions in proliferation of certain cell lines when treated with lycopene. This study used seven human cell lines to measure the effect of lycopene on cell proliferation across normal human plasma concentrations of lycopene. Seven cell types, cancerous and noncancerous, were treated with lycopene from 0.0001 to 10 microM for 24, 48, and 72 h and counted electronically. Controls and experimental samples were compared using the Mann-Whitney U-test at a 95% confidence level. All cells grew normally and there was no significant difference between any of the controls. The Hep-G2, liver adenocarcinoma cell line, showed a reduction at the high doses after 24 h and the IMR-90, noncancerous lung cell line, showed a reduction at the highest dose after 72 h when compared to the solvent control. The A431, skin carcinoma, DU-145, prostate carcinoma, HS-68, noncancerous skin, A549, lung carcinoma, and HS-578T, breast carcinoma, all showed no reduction in proliferation. This indicated that lycopene at the physiological range does not significantly affect cell proliferation in an in vitro model and requires more careful investigations.