A multiple-stage process for the conversion of a heavy hydrocarbonaceous charge stock, contaminated by the inclusion of sulfurous and nitrogenous compounds, into lower-boiling hydrocarbon products. Fresh feed and hydrogen are introduced into a catalytic hydrorefining reaction zone to convert the contaminants into ammonia and hydrogen sulfide. Hydrorefined product effluent is admixed with the effluent from a catalytic hydrocracking reaction zone, and separated into various product streams. Hydrocarbons boiling above the predetermined end boiling point of the desired end product and hydrogen are introduced into the catalytic hydrocracking reaction zone for conversion to lower-boiling hydrocarbons. Hydrocracked effluent is admixed with the hydrorefined effluent as aforesaid.