American Broadcasting Cos. Inc., which is leading CBS and NBC in the prime-time television ratings, reported yesterday that it had record earnings and revenues in the fourth quarter and full year of 1977.

Earnings for the year soared to $109.79 million ($6.04 a share) from $71.747 million ($4.05) in 1976. Revenues climbed to $1.616 billion from $1.342 billion.

ABC's fourth-quarter profits reached $34.645 million ($1.91) from $24.397 million ($1.36) a year earlier. Sales were up to $499.8 million from $391.2 million.

Chairman Leonard H. Goldenson and President Elton H. Rule said that in 1977 - ABC's 25th year - "the ABC Television Network took its place as the world's largest advertising medium."

Allied Chemical Corp. yesterday reported that 1977 earnings increased by 15 percent to $135.3 million ($4.82 a share) from $117.2 million ($4.19) in 1976 Sales for 1977 reached a record $2.9 billion, up 11 percent from $2.6 billion in 1976.

The 1977 results were $3.2 million, or 12 percent a share, higher than the preliminary unaudited income figure released on Jan. 19.

Results for both 1976 and 1977 reflected extraordinary charges related to production of the suspected carcinogen kepone at Hopewell, Va., and nonrecurring items.

Chairman John T. Connor said independent accountants' opinion on the financial statements would be qualified "with respect to possible liabilities arising out of the production of Kepone as well as a claim for damages by Armco Steel Corp. for failure to deliver contractural quantities of coke."