John Muir & Co., which wowed Wall Street by underwriting an impressive 43 new stock issues in jut 18 months, recently had been belted by a series of class-action lawsuits brought by some investors in the new issues.

In the lastest suit, filed earlier this week, Muir and some of the firm's prinicpals are accused of being part of a scheme to use proceeds from the sale of one new issue to buy other new issues brought out by Muir.

Filed in U.S. District Court here, the suit was brought by Nacny Spector, a stockholder of Cayman Islands Reinsurance Corp. who claims that the company became "an investment tool of Muir."

Spector says that a portion of the $4.8 million raised in the sale of stock in the West Indies company to the public last December was used to prop up the prices of several of Muir's underwritings and did not go into the company's working capital as the offering prospectus had promised.

The funds allegedly were invested in Basic Earth Science ysytems Inc., Security America Corp., Brady Energy Corp. and Aneco Reinsurance Co., all new issues brought by Muir.

Raymond L. Dirks, the general partner of Muir who is among those named in the suit, calls the suit "frivolous." Dirks says a Muir executive did suggest investments to Cayman Islands Reinsurance Chairman Michale Scott but that Cayman Islands Reinsurance "itself made all the investment decisions."

The Securities and Exchange Commission office here is investigating several of Muir's stock underwritings, including Cayman Islands.

Meanwhile, the SEC in Washington is conduction a seperate investigation into another Muir underwriting, Brady Energy in Ft. Lauderdale.

Last December, Muir sold the public a new issue of perferred stock in Brady Energy, but in February 140,000 shares of the stock ended up back with Muir when an investor's check bounced.

Muir already had passed along the proceeds from the sale to Brady Energy. But F. Lee Bailey, an attorney for company founder Ralph Brady, claims that Muir threatened to cancel the whole underwriting if Brady did not find a buyer for those shares.

Then, an Austin company, Trans National Energy Co., bought 100,000 of the shares. But apparently the company paid for the stock with $1 million paid it by Brady Energy -- funds that had been raised by the sale of the preferred stock and that were supposed to finance a joint drilling venture.