The owner of the Galaxy Chemical Co. testified here today that his firm had been having its most profitable year ever in 1974 until the national media focused attention on the abnormally high number of cancer deaths in the north-eastern Maryland valley where the plant was located.

The small chemical reprocessing firm had made about $22,000 in the first eight months of 1974, owner Paul J. Mraz testified. But after the stories about the cancer death studies of Dr. Pietro U. Capurro were published, he said, business dropped dramatically, and the firm ended up the year with a net loss of more than $9,000.

Mraz admitted on cross-examination that Galaxy continued to show a profit for a month or more after the initial onslaught of negative publicity in national newspaper and magazines.

Mraz and the Galaxy firm are suing Cappuro for a total of $2.1 million, claiming that the Italian-born doctor - a pathologist at the Union Hospital in Elkton, Md. -defamed Galaxy by falsely linking the firm's omissions to cancer deaths in the Little Elk Valley in Cecil Country.

Capurro and his attorneys have answered that the doctor's two studies of cancer deaths in the area never specifically blamed Galaxy for the cancer problem. Neither side in the suit has contested the fact that there were many more cancer deaths in the Little Elk Valley than might have been expected between 1967 and 1976 - including an extremely high number of lympathic cancers.

During the firm's 15 years of existence, Mraz testified yesterday, a total of slightly more than 8 million gallons of chemicals were processed and recycled there, the bulk of the volume coming after the plant started fulltime operations in 1965.

Mraz also testified that Galaxy showed a profit in only six of the years that is operated. When Capurro's attorney cross-examined Mraz, he pointed out that with the exception of one year, the firm never paid any U.S. income tax.