Business-to-business e-commerce companies saw their shares tumble Friday following a weak performance by one of the smaller players in the sector.

Leading the rout was

Ventro

(VNTR) - Get Report

, the business-to-business online marketplace operator whose shares plunged 30% Friday morning after analysts downgraded the stock, citing disappointing second-quarter transaction growth.

Of the 10 firms that follow the stock, six --

Prudential

,

Deutsche Banc Alex. Brown

,

Credit Suisse First Boston

,

Robertson Stephens

,

Wedbush Morgan

and

First Union Securities

-- cut their ratings. (Of those firms, Prudential and Robertson Stephens have been Ventro underwriters.) Ventro shares slid 6 3/16 to 14 3/16 by late morning, and shares in Ventro's larger competitors also fell, by 4% to 8%, as the tech-crazed

Nasdaq

index pulled back from Thursday's rally.

Not Enough Transactions

Analysts are concerned because Ventro's revenue from transaction fees fell some 30% short of estimates, totaling $1.2 million. Overall, the company reported $1.8 million in net revenue, but that number was bolstered by $600,000 in services revenue that analysts hadn't expected to see.

The company told

TheStreet.com

Thursday night that the slow integration of B2B technology at big corporations was hindering the ramp-up of transactions over its exchanges. That caused a lower-than-expected total transaction volume of $29.1 million in the quarter. Analysts were hoping for around $35 million.

David Perry, Ventro's president and chief executive, told

TheStreet.com

that investors shouldn't overlook the good news in the most recent quarter's results. "We're finding more and more ways to make money," Perry said Thursday night.

Numerology

All the same, Ventro's results look particularly bad in the context of the strong numbers other B2B companies reported. Larger competitors such as

Ariba

( ARBA),

Commerce One

( CMRC) and

PurchasePro.com

(PPRO)

all beat analysts' revenue expectations by wide margins.

Ventro posted a 74 cent-a-share loss Thursday, beating analysts' expectations by 6 cents. But among fast-growing, money-losing B2B companies, the emphasis is on revenue, not earnings.

The company went public a year ago at $15 share as

Chemdex

, but changed its name to Ventro earlier this year when it decided to branch out from its core life sciences electronic marketplace. Since then, it has also focused on the medical industry, through its

Promedix

site, and on fluid processing materials, through its

Industria Solutions

marketplace. But that expansion hasn't helped grow transactions quickly, and 97% of Ventro's business is still going through the Chemdex site.

Its shares, now in the midteens, traded as high as 243 1/2 at the height of the B2B frenzy earlier this year.