Treatment of a light fraction of petroleum treated by metallic catalysis results in a liquid mixture of low molecular weight compounds named LarimshTM (LR). Acute and chronic topical treatment of mice with LR (85 days with 0.1 to 1 mL/animal) indicated no signs of toxicity other than a non dose-dependent, reversible alopecia appearing by the 4th week of topical application. Alopecia completely reversed 2 weeks after treatment withdrawal. Acute oral LR (0.001 to 1 mL; the lowest doses diluted in corn oil as vehicle) gave an estimated LD50 of 21 g/kg (C.I. 95 %: 10.94-41.2 g/kg. LR density = 0.867 g/mL). The antineoplastic action of LR was observed using combined oral and topical treatments in mice; inoculated with a lymphoid leukemia cell line in ascitic phase (International Registry L5178Y); and in terminal patients with prostate cancer (TPCA)--who agreed to receive LR as a compassionate treatment. The survival time for mice was significantly increased when compared with non-treated inoculated mice (51 +/- 2 versus 38 +/- 2 days, p < 0.05, mean +/- SD, N = 6 per group). In 15 patients, LR treatment for 5.5 months (C.I. 95 %: 2.9 to 8.0 months) significantly increased the expected survival time diagnosed to TPCA by their treating physicians (C.I. 95 %: 2.2 to 5.4 versus 12.6 to 41.2 months, p < 0.05) which occurred concomitantly with a significant reduction of blood levels of total prostatic antigen (average 94.5%, range: 67.3 to 99.9%). A theoretical proposal is advanced as a likely explanation of LR actions.