It is the latest in a series of lawsuits by pharmaceutical firms worried that Japanese companies are too easily able to get around patent legislation.
SmithKline's action claims Y4.9bn (pounds 31m) from Fujimoto and Towa for producing and selling copies of cimetidine, the active ingredient in Tagamet, a best-selling anti-ulcer drug.
A SmithKline spokesman said the company was seeking Y3bn from Fujimoto for loss of profit since the Japanese company launched its generic version of cimetidine in 1986. Towa is being sued for Y1.9m.
The suit against Fujimoto was filed on 30 June. The Towa suit was filed yesterday. SmithKline had already obtained an injunction in the Tokyo district court stopping Towa from manufacturing and selling its cimetidine generic drug.
One pharmaceuticals analyst said: 'Drug companies are bemoaning the behaviour of the Japanese. There is concern that if they can buy the raw ingredients abroad they can somehow sidestep the patent legislation.
'SmithKline's action is the first time we have seen a company actually put a figure on what it has lost.'
Earlier this year Sandoz, the Swiss drugs company, obtained injunctions against seven Japanese generic manufacturers to protect an anti-asthmatic agent. It has several ongoing patent disputes with other companies.
Last year Sanofi, the French company, took out provisional injunctions against eight generic manufacturers, including Towa, over alleged infringements of patent rights on its drug Panaldine.
There has been concern about generic manufacturers for some years. But the recent higher price of new drugs under the Japanese drug tariff system has forced overseas companies selling into Japan to act to protect their patents.
Tagamet, launched by SmithKline in 1976, has broken the pounds 1bn sales barrier in each of the past seven years.
In a separate issue Barr Laboratories, one of two US generic companies which this week lost a patent battle with Wellcome, the British pharmaceutical group, is to appeal.
Wellcome won a court ruling against Barr and Novopharm to protect the patent of AZT, also called Retrovir, its treatment against the Aids virus HIV.Reuse content