Boosted by improved television ratings, the American Broadcasting Companies, Inc., yesterday reported a sharp turnaround in 1976's fourth-quarter and full-year profits.

In the last quarter, ABC said it earned $24.39 million compared with a $3.95 million loss in 1975.

"Nineteen seventy-six was a great year for the industry," said ABC spokesman Jim Abernathy. "Our sales finally started reflecting in the network's better ratings which started to show up in the last quarter of 1975."

ABC, once an also-ran in the race for prime-time ratings, currently holds the top ranking "by a wide margin," Abernathy said. The company had complained in 1975 of declinging TV renevue.

Both of ABC's main competitors, RCA Corp. and CBS, Inc., also showed improved profits last year, and industry analysts say the trend should continue as advertising rates continue to climb.

ABC's fourth-quarter profits equaled $1.36 a share. Revenue in the period gained from $321.75 million in 1975 to $391.23 million last year.

For all of 1976, ABC earned $71.75 million ($4.05 a share) compared with $17.10 million (99 cents) in 1975. Full-year revenue rose from $1.07 billion to $1.34 billion.

ABC said 1975 results were depressed by losses on the company's investment in a now-closed Wildlife preserve in Largo.

In Los Angeles, meanwhile, the giant entertainment conglomerate, MCA, Inc., said full-year income fell from $95.51 million ($5.35 a share) in 1975 to $90.23 million ($5.02) last yaar. Revenue also fell, from $811.48 million in 1975 to $802.92 million in 1976. Income in 1975 got a boost from MCA's hit movie, "Jaws," but analysts say the company still retains excellent earnings potential.

In addition to film and television production, MCA makes records and tapes, publishes books and music, sells novelty and gift merchandise, runs various recreation facilities, and owns a savings and loan association.