Approximately 50 Massachusetts service stations have begun test-marketing gasohol, a mixture of alcohol and gasoline, which proponents say will bring better gas mileage and less pollution.

Three companies--Gibbs Oil, Texaco, and Top Gas-are offering the product in Massachusetts.

Most of the stations began selling gasohol in recent months as a way to make up for lower allotments of gasoline available to them and to "get in on the ground floor of the gasohol market," Chris Hansen, assistant to the director of resource development at the State Office of Energy Resources, said yesterday.

At the 23 Boston-area Texaco stations which sell gasohol at an average price seven cents higher than the price of unleaded gasoline, gasohol comprises about 15 per cent of total gasoline sales, Edward V. Lewandowski, Texaco's New England resale director, said yesterday.

Lewandowski said Texaco is test- marketing gasohol in Boston, Indianapolis and New York for at least six months. Texaco's program began in Boston on October 9, he added.

Ross L. Pusatere, vice president for marketing of Gibbs Oil, a New England distributor, said yesterday his company has been selling gasohol for about three cents more per gallon than unleaded as "a get-acquainted offer" in 11 stations. "So far the response has been very favorable," he added.

Lewandowski said responses to a questionnaire Texaco has distributed to gasohol purchasers indicate that the consumers buy the product because they are reducing America's dependence on foreign oil and improving the performance of their automobiles.

Gasohol is 10 per cent ethanol, an alcohol which can be distilled from such materials as grain and wood pulp, and 90 per cent unleaded gasoline. The blend has a slightly higher octane rating than regular unleaded gasoline.

Hansen said ethanol currently costs about $1.61 a gallon, but that experts working on increasing the efficienty of the distillation process predict the cost will eventually be 85 cents a gallon or lower.